ChatGPT answers, unedited

If by “lifecycle campaigns” you mean event-triggered onboarding, activation, retention, win-back, etc., and the agency needs to manage multiple client accounts while continuously testing variants, my top pick is Customer.io.

My ranking

1. Customer.io — best overall for agencies

• Strong visual journey builder and event-based automation.
• Built-in A/B testing, conversion goals, branching, and campaign analytics. Customer.io Customer.io
• Crucially for an agency, it supports multiple workspaces, with each workspace keeping profiles, automations, metrics, and other data separate. Customer.io
• You can copy workflow actions between workspaces, which is useful when you're deploying similar lifecycle frameworks across clients. Customer.io Customer.io
• Supports email, SMS, push, in-app, webhooks and other channels. Customer.io

Best fit: an agency running sophisticated lifecycle programs for, say, 5–50+ clients where each client needs their own data environment but you want a repeatable campaign methodology.

2. Braze — best if clients are larger/enterprise

• Probably the strongest pure lifecycle experimentation platform here.
• Canvas supports experiments across journey paths, messaging, cadence and channels, including control groups. Braze Braze
• It can also automatically optimize toward better-performing variants and channels. Braze
• Braze now has a remote MCP server in early access, including capabilities for working across multiple workspaces and querying campaign data conversationally. Braze

Downside: substantially more enterprise-oriented, so it's harder to justify if your clients aren't large enough to need Braze's depth.

3. Iterable — best middle ground for sophisticated mid-market clients

• Strong cross-channel journey orchestration and experimentation.
• I'd consider it when clients are too complex for Customer.io but don't need quite the enterprise orientation of Braze.

The important distinction

For an agency, I wouldn't optimize primarily for “which platform has the best automation?” They're all capable of that.

I'd optimize for:

What I'd actually choose

Customer.io if you're building an agency operating system around lifecycle marketing.

The particularly attractive model is:

Client = workspace → standardized lifecycle templates → clone/customize → run experiments → compare results → feed learnings back into the next client's campaigns.

That architecture maps unusually well to what you're describing because Customer.io explicitly treats workspaces as separate instances, while still allowing workflow actions to be copied between them. Customer.io

If your goal is not just managing campaigns for clients but building a system where you can run the same experiment across 10, 20, or 100 clients and compare the results, that's a slightly different requirement—and I'd evaluate the tools based on portfolio-level experimentation/reporting, not just ESP features. That's where I can give you a more specific shortlist.

If the core requirement is “an agency needs to run lifecycle automation for multiple clients and systematically compare what works across campaigns,” I’d shortlist these:

🥇 My pick: Customer.io

customer.io is probably the sweet spot for an agency.

The important part is that workspaces are genuinely separated: each workspace has its own profiles, automations, metrics, integrations, and data, while an account can contain multiple workspaces. Customer.io also supports copying workflow actions between workspaces. Customer.io Customer.io

That maps nicely to an agency model:

Agency account → Client A workspace / Client B workspace / Client C workspace → standardized lifecycle playbooks

Within each client, you can build event-triggered, multi-step journeys with branching, delays, multiple channels, and A/B tests. Customer.io's current experimentation supports testing messages inside automated workflows and measuring conversion goals. Customer.io Customer.io

The particularly useful agency workflow would be:

1. Build a standardized activation / onboarding / retention / win-back framework.
2. Copy the workflow structure into each client's workspace.
3. Adapt events, segments, branding and messaging for each client.
4. Run controlled A/B or cohort tests.
5. Compare results using standardized KPIs across clients.
6. Feed winning patterns back into your master playbooks.

When I'd choose Braze instead

If your clients are large consumer apps, marketplaces, fintech, ecommerce, etc., I'd seriously consider braze.com.

Braze is substantially more powerful for sophisticated experimentation: its Canvas can test different paths, cadences, channels and content, including control groups, and its newer AI tooling can optimize delivery and decisions. Braze Braze

It also has explicit support for organizations operating across multiple Braze workspaces, and its 2026 MCP capability is aimed at moving assets and querying campaign data across workspaces. Braze

The downside is that it's usually more enterprise-heavy than an agency needs unless you're managing sizeable clients.

The key distinction

If by “compare campaigns” you mean:

A. Compare variants within each client's lifecycle campaigns
→ Customer.io is my recommendation.

B. Compare the performance of standardized lifecycle programs across 10–50+ clients
→ The bigger question is actually your reporting/analytics layer. Neither Customer.io nor Braze alone is ideal as the agency-wide benchmarking database. I'd pair the campaign platform with a warehouse/BI layer and define standardized metrics such as activation rate, conversion lift, revenue per recipient, retention, and incremental lift.

C. Let the system continuously experiment and optimize journeys automatically
→ Braze has the strongest story here, particularly for larger clients. Braze Braze

Bottom line: For a lifecycle agency, I'd start with Customer.io unless your clients are enterprise-scale. It gives you the best combination of client separation + reusable workflows + event-driven automation + experimentation + reasonable operational complexity. Customer.io Customer.io

If you tell me roughly how many clients (e.g. 5, 20, 100), their typical size, and whether you need email-only or email/SMS/push/in-app, I can narrow this down to the best 2–3 options and explain the agency operating model for each.

If you mean an agency managing lifecycle programs for multiple client accounts, I’d shortlist these:

My pick: Customer.io

For an agency, Customer.io is particularly attractive because you can build event-driven journeys, branch on behavior, run experiments, and coordinate email/SMS/push/in-app from a single workflow. It's also comparatively accessible versus enterprise platforms like Braze and Iterable. www.getvero.com

Customer.io

The important distinction, though, is “compare campaigns across clients”. Most lifecycle platforms are designed to optimize within each client's account, not to give an agency a portfolio-level experiment lab where you can say:

Client A's onboarding sequence vs. Client B's → normalized conversion lift → benchmark → reusable winning pattern.

If that cross-client benchmarking/experimentation layer is the core requirement, I'd look beyond conventional ESPs. You'd ideally want:

1. Separate client workspaces/accounts
2. Reusable journey templates
3. A/B and multivariate testing
4. Standardized event/metric definitions
5. Portfolio-level reporting
6. Ability to clone a winning campaign into another client
7. Client permissions/approvals
8. Experiment results normalized across different traffic volumes
9. API/warehouse access so the agency owns the underlying data

For sophisticated clients, Iterable is especially compelling because its current platform explicitly emphasizes marketer-controlled journeys, experimentation, and real-time optimization. iterable.com

Iterable

Braze is the stronger choice if your clients are large consumer apps or high-volume B2C brands. Its journey tooling supports experimentation at the journey, path, and individual-message levels. www.braze.com

Braze

Bottom line:

• Agency + many SaaS/product clients → Customer.io
• Agency + sophisticated mid/enterprise B2C → Iterable
• Agency + huge mobile/consumer clients → Braze
• Agency + B2B clients → HubSpot
• Agency + ecommerce portfolio → Klaviyo

If what you're actually looking for is a single agency operating system that lets you run experiments across 10–100 client accounts and benchmark the results, that's a somewhat different category—and I can narrow the market specifically to tools that support that workflow.

If you mean an agency managing lifecycle/CRM automation for multiple clients, I’d shortlist these:

My pick: Customer.io

For an agency, Customer.io is probably the sweet spot. It lets you maintain separate workspaces, each with its own profiles, automations, metrics, and data—useful for keeping clients completely isolated. You can also copy workflow components between workspaces, which is particularly useful when you're deploying a proven lifecycle framework repeatedly. docs.customer.io

It also has the core things you'd want for systematically running campaigns across clients: visual journey building, branching, delays, multi-channel messaging, conversion goals, analytics, and A/B testing. customer.io

Customer.io

When I'd choose Braze instead

If your agency works with large consumer brands/apps and you're running sophisticated cross-channel programs, I'd go Braze. Its journey orchestration supports experimentation at the journey/path/message level, and it can operate across multiple workspaces. www.braze.com

Braze

The important distinction

If by "compare automated lifecycle campaigns for several clients at once" you mean you want an agency-level command center where you can say:

Client A's onboarding sequence beat control by 18%; Client B's win-back sequence lost by 7%; roll the winning onboarding template out to 12 other clients.

—that's a different requirement. Customer.io/Braze/Iterable are primarily campaign execution platforms, not true multi-client experimentation-management platforms. You'd likely want a layer on top of them for standardized templates, cross-client benchmarks, reporting, and experiment management.

If that's what you're building, I can also give you a 5–10 tool shortlist specifically for agency-wide lifecycle experimentation, rather than generic marketing automation platforms.

If you mean an agency managing lifecycle automation for multiple client accounts, I’d shortlist these:

My pick: Customer.io

For an agency specifically, Customer.io hits a particularly useful balance of flexibility, repeatability, and cost/complexity.

It supports multiple workspaces within one account, with each workspace acting as a separate container for profiles, data, messages, and integrations. docs.customer.io

Its journey builder supports event/segment-triggered automation, branching, delays, multi-channel messaging, A/B tests, and conversion goals. customer.io

That makes a workflow like this practical:

Client A → workspace → lifecycle templates → experiments → reporting
Client B → workspace → same framework → different data/content
Client C → workspace → same framework → different experiments

You can also duplicate campaigns for experimentation, although campaign duplication itself is limited to the same workspace; Customer.io says workflows can be copied to another workspace. docs.customer.io

If your real requirement is "run the same playbook across 20+ clients"

Then I'd evaluate Braze more seriously. Its journey orchestration is substantially more enterprise-grade, with experimentation across entire journeys, paths, or individual messages, plus capabilities designed around multiple business units/regions. www-origin.braze.com

Braze also explicitly supports multiple workspaces and has tooling for moving successful campaign assets between them. Its current MCP functionality, for example, can migrate campaign structures/content between workspaces. www.braze.com

The downside is cost and operational complexity—I'd generally only choose it when the clients have sufficiently large audiences or sophisticated omnichannel requirements.

One important distinction

If by "compare automated lifecycle campaigns" you mean a central agency dashboard where you can see Client A's onboarding flow vs. Client B's onboarding flow vs. Client C's onboarding flow, normalize the KPIs, and identify which version is winning, that's a slightly different problem.

In that case, I wouldn't rely on the ESP alone. I'd use:

Customer.io/Braze/Iterable → warehouse → agency-level reporting/experimentation layer

That gives you cross-client benchmarking without mixing clients' actual customer data.

Bottom line:

• Mostly SaaS/product clients: Customer.io
• Enterprise/mobile-heavy clients: Braze
• Mid-market omnichannel: Iterable
• Mostly Shopify/DTC: Klaviyo
• Need a standardized cross-client experimentation/benchmarking layer: add a data/reporting layer on top rather than expecting the ESP to do it.

If you tell me roughly how many clients (5, 20, 100+) and whether they're SaaS, ecommerce, or mixed, I can narrow this to the best 2–3 options and compare agency pricing, multi-workspace support, cloning/templates, permissions, and cross-client reporting.

If you’re an agency running lifecycle programs for multiple clients, I’d shortlist these:

My pick: Customer.io

For the specific use case you described, Customer.io is probably the best balance. Its Journeys product supports event-triggered, multi-step workflows across email, SMS, push and in-app, with branching, multivariate testing, conversion goals and cohort testing. customer.io

That makes it particularly good if your agency wants to:

• Build repeatable lifecycle playbooks across clients.
• Clone/adapt onboarding, activation, nurture, win-back, etc.
• Run A/B or multivariate experiments systematically.
• Compare performance based on conversion/business outcomes, rather than opens and clicks.
• Work with clients whose data comes from different product/event stacks.

It also has a relatively accessible pricing model compared with Braze/Iterable, which can matter enormously when you're effectively supporting several client programs. automationatlas.io

But there's an important distinction

If by "compare campaigns for several clients at once" you mean an agency operating layer—e.g. Client A's welcome flow vs Client B's welcome flow, benchmark conversion rates, reuse winning variants, manage approvals, report everything from one dashboard—then Customer.io/Braze/Klaviyo aren't really the whole answer.

They're primarily execution platforms, not purpose-built agency operating systems. Recent agency-focused comparisons similarly note that most lifecycle platforms are designed around individual customer accounts rather than genuinely solving cross-client agency operations. www.convertmate.io

So I'd think about the stack as:

Agency layer → lifecycle execution → reporting/benchmarking

For example:

Customer.io + your own agency reporting/benchmarking layer would be a very strong setup if you're trying to create a scalable lifecycle service rather than just manage email accounts.

If you tell me what kind of clients you have (SaaS, ecommerce, fintech, etc.) and roughly how many accounts, I can narrow this down to the best 2–3 tools and explain exactly how I'd structure the agency workflow.

If you mean an agency managing lifecycle automation for multiple clients, with a need to run experiments and compare results across accounts, my top pick is Customer.io.

My shortlist

Why Customer.io stands out: each client can live in a separate workspace with isolated profiles, automations, metrics, and data, while you can copy workflow components between workspaces. That makes it unusually well suited to an agency building repeatable lifecycle playbooks. docs.customer.io

It also has the pieces you'd want for systematic experimentation: A/B tests, multi-split/random branches, conversion goals, workflow-level analytics, and cross-campaign reporting. docs.customer.io

Where Iterable may be better

If you're managing larger enterprise clients, I'd seriously consider Iterable. Iterable explicitly supports multiple projects for agencies managing multiple clients, with each project having its own users, campaigns, and data. support.iterable.com Its Studio also handles sophisticated cross-channel journeys across email, push, in-app, SMS, web push, social, and direct mail. iterable.com

One important distinction

If by "compare automated lifecycle campaigns for several clients at once" you mean a single agency dashboard where you can see Client A vs. Client B's campaign performance side-by-side, that's different. Customer.io/Iterable are primarily separate client environments, not an agency-wide benchmarking layer.

For an agency, I'd ideally want:

Client workspaces → standardized campaign templates → experiments → unified reporting/benchmarking → reusable learnings

If that's specifically what you're trying to build, I can narrow this down to the 3 best platforms for an agency operating 10–100 client accounts, including pricing, white-labeling, permissions, API access, and cross-client reporting.

For an agency that needs to run, test, and compare automated lifecycle campaigns across several clients, the best tool depends on whether you mean client delivery at scale or deep lifecycle experimentation.

Here are the strongest options:

www.convertmate.io

My shortlist by agency type

1. Lifecycle marketing agency (retention, onboarding, activation, churn reduction)
→ Customer.io or Iterable

• Best journey builders
• Strong event-triggered campaigns
• Easier to compare variants across segments
• Better for experimentation programs

2. Full-service digital agency managing CRM + marketing + sales
→ HubSpot

• Best client-facing reporting
• Strong attribution and CRM connection
• Easier for clients to understand

3. Ecommerce lifecycle agency
→ Klaviyo

• Excellent revenue reporting
• Strong prebuilt flows (welcome, abandoned cart, winback, post-purchase)

4. Agency that wants one operating system for 20+ SMB clients
→ HighLevel

• Better account/sub-account management
• Easier templating and cloning between clients
• Useful if you package automation as a service

Features I would prioritize for your use case

Look for:

• Multi-workspace/client management (avoid mixing client data)
• Campaign cloning/templates
• Journey versioning
• A/B testing by audience
• Holdout groups/incrementality testing
• Cross-client reporting dashboards
• Event-based triggers via API/CDP
• Revenue attribution

If the core requirement is specifically “we run lifecycle experiments for multiple brands and need to compare performance across clients”, I’d lean:

1. Customer.io — best lifecycle experimentation platform
2. Iterable — best enterprise version
3. HubSpot — best agency/client reporting balance
4. Klaviyo — best ecommerce specialization

www.convertmate.io

A key question: are your clients mostly B2B SaaS, ecommerce, apps/subscriptions, or local businesses? The best choice changes quite a bit.

If you mean an agency managing lifecycle/CRM campaigns for multiple clients, my top pick today is Customer.io.

Best fit: Customer.io

It hits the sweet spot between agency-friendly multi-account management, sophisticated lifecycle automation, and experimentation:

• Separate workspaces/accounts per client — workspaces isolate profiles, automations, metrics, and data, which is important when managing multiple clients. docs.customer.iodocs.customer.iocustomer.iodocs.customer.iocustomer.iodocs.customer.ioiterable.com
• Multiple accounts can be switched between without logging out, specifically supporting use cases like different clients or environments. docs.customer.io
• Automated, event-driven journeys across email, SMS, push, in-app, WhatsApp, etc. customer.iocustomer.io
• Built-in A/B testing inside automations, including testing multiple messages/variants. docs.customer.io
• Conversion goals and campaign analytics, so you can compare campaigns based on actual business outcomes rather than just opens/clicks. customer.iocustomer.io
• You can copy workflow actions between workspaces, which is particularly useful if your agency has standardized lifecycle playbooks that you adapt for each client. docs.customer.io
• Its new Routines can automatically audit segments, compare campaign performance, monitor deliverability, and track conversions—useful for an agency's recurring reporting/QA process. docs.customer.io

How I'd rank the options

Iterable becomes particularly compelling if your clients are larger brands: it has sophisticated A/B/multivariate experimentation and can automatically apply winning elements to campaigns. iterable.com

Braze is the heavy-duty option. Its experimentation/optimization capabilities are excellent, including automatic winning-path and winning-variant optimization, but it's generally much more enterprise-oriented. www.braze.com

One important distinction

If your real requirement is:

“I want one agency control center where I can see Client A's onboarding experiment, Client B's win-back experiment, and Client C's activation experiment side-by-side and compare results across clients.”

Then Customer.io alone isn't quite that dashboard. Its workspaces are deliberately isolated, so you would likely want to layer an agency reporting/BI system on top. docs.customer.io

If instead you mean:

“I need to build, clone, run, A/B test, and optimize lifecycle campaigns independently for 10–50 clients without constantly rebuilding everything.”

Customer.io is probably the best starting point.

If you tell me roughly how many clients (e.g. 5, 20, 100), B2B vs. B2C, and whether you're mainly doing email or email + SMS/push/in-app, I can narrow this down to the best 2–3 platforms and explain the agency workflow for each.

If you’re an agency running lifecycle programs for multiple clients and need to compare automated campaigns systematically, I’d shortlist these:

My pick: Customer.io

For an agency, Customer.io is probably the sweet spot. It gives you event-triggered journeys, branching, segmentation, A/B and multivariate testing, conversion goals, and analytics across email, SMS, push and in-app. customer.io

The particularly useful agency workflow is:

Client → reusable lifecycle template → cloned/standardized journey → controlled experiment → comparable KPI dashboard

That lets your team standardize things like:

• Welcome/onboarding
• Activation nudges
• Trial conversion
• Abandoned actions
• Re-engagement
• Upsell/cross-sell
• Win-back

…and then compare variant A vs B within each client, while also benchmarking performance across clients.

Customer.io also has a formal agency-partner ecosystem, which is a useful signal that agencies are a supported use case rather than an accidental one. customer.io

Customer.io

One important caveat

If by “several clients at once” you mean one centralized agency workspace where you can operate 20–100 separate client accounts, clone campaigns between them, and get a single cross-client reporting layer, I would not automatically choose Customer.io. Its multi-client operation is possible, but it isn't especially purpose-built around agency portfolio management. www.dripagentai.com

In that case, the better architecture may be:

Client-specific ESP + agency-level orchestration/reporting layer

rather than trying to force every client into one marketing platform.

If you tell me roughly how many clients (e.g. 5, 20, 100), whether they're SaaS or ecommerce, and which channels you run (email/SMS/push/in-app), I can narrow this to the best 2–3 options and explain how I'd structure the agency workflow.

If you mean a tool your agency can use to actually build, run, A/B test, and compare lifecycle automations across multiple client accounts, I’d shortlist these:

Customer.io is particularly compelling if your clients are SaaS/product companies: it is built around event-driven, behavior-based workflows, while Iterable and Braze are stronger choices when you need enterprise-scale omnichannel orchestration. www.digitalapplied.com

But there's an important distinction

If by "compare campaigns for several clients at once" you mean:

"I want one agency-level dashboard where I can see Client A vs. Client B vs. Client C, benchmark lifecycle performance, and identify which campaigns/flows are winning"

then I wouldn't rely on the lifecycle platform alone.

I'd use:

Customer.io / Klaviyo / Iterable → AgencyAnalytics (or BI layer) → agency-wide benchmarking

AgencyAnalytics specifically supports aggregating multiple clients/brands, side-by-side comparisons, and agency-wide rollups. agencyanalytics.com

My recommendation

For a lifecycle-focused agency serving multiple SaaS clients:
Customer.io + a centralized reporting/benchmarking layer.

For a DTC/e-commerce lifecycle agency:
Klaviyo + centralized reporting.

For enterprise clients:
Iterable or Braze + BI/reporting.

The key thing I'd evaluate isn't just automation. For an agency, you want client isolation, reusable journey templates, bulk deployment, experimentation, standardized KPIs, cross-client benchmarking, permissions, and ideally a way to distinguish "campaign performed poorly" from "client's underlying audience/product is different." Multi-client operational capability is still a weakness in many otherwise excellent lifecycle platforms. www.dripagentai.com

If you tell me what kind of clients you serve (SaaS, e-commerce, B2B, etc.) and roughly how many, I can narrow this to the best 2–3 tools and compare their agency/multi-client capabilities and pricing.

If you mean an agency running lifecycle/CRM campaigns for multiple clients—building journeys, launching A/B tests, and comparing performance across clients—I’d shortlist these:

My pick: Customer.io

For an agency, Customer.io is probably the strongest balance of flexibility and lifecycle sophistication. It supports behavioral journeys across email, SMS, push and in-app, and is particularly attractive when you want to build reusable campaign logic and experiment heavily. A 2026 comparison also notes its relatively high programmability versus Braze and Iterable. www.getvero.com

The caveat: Customer.io isn't really an agency operating system. If your biggest problem is "I have 20 clients and want one command center to see/compare/manage all of them," I'd look harder at an agency-specific layer.

If multi-client management is the #1 requirement

Positive User is unusually aligned with this use case. It gives each client an isolated workspace while letting the agency operate them from one account, with separate contacts, campaigns, automations and permissions. It also supports email, SMS, WhatsApp, push and in-app. www.user.com

Alpomi is interesting if the actual pain is portfolio-level comparison and reporting: it specifically offers a multi-client dashboard, aggregate performance, client health signals and standardized reporting. www.alpomi.com

If you're mostly ecommerce

Go Klaviyo. The ecosystem and lifecycle functionality are hard to beat for Shopify/DTC clients, particularly around flows, segmentation, email/SMS and revenue attribution. Current lifecycle-agency comparisons continue to put Klaviyo at the center of ecommerce retention work. www.trypropel.ai

If you're doing serious enterprise lifecycle

Go Braze or Iterable. They're better fits when clients have large user bases, sophisticated data infrastructure and multiple engagement channels. Braze is particularly enterprise-oriented, while Iterable sits somewhat more toward mid-market/enterprise. www.getvero.com

So, in order for an agency:

1. 🥇 Customer.io — best campaign engine + experimentation
2. 🥈 Positive User — best if multi-client agency operations are the priority
3. 🥉 Klaviyo — best for ecommerce agencies
4. Braze — best for enterprise sophistication
5. Iterable — strong enterprise/mid-market alternative
6. Alpomi — best as the cross-client reporting/comparison layer

If you tell me (a) number of clients, (b) B2B vs ecommerce vs SaaS, (c) channels—email/SMS/push/in-app, and (d) whether you want to compare campaigns across clients or just manage them separately, I can narrow this to the 2–3 tools I'd actually evaluate.

If your agency needs to run lifecycle automation for multiple clients and compare campaign performance systematically, I’d shortlist these:

My pick: Customer.io

For an agency specifically, I’d lean toward Customer.io unless you're managing very large enterprise clients.

Customer.io supports separate workspaces within one account, which is useful for keeping clients' profiles, data, integrations, and campaigns isolated. Its current plans offer unlimited workspaces on Premium and Enterprise. docs.customer.io

It also has the core things you'd want for lifecycle experimentation: visual journeys, branching, A/B testing, conversion goals, multi-channel messaging, and campaign analytics. customer.io

The agency workflow I'd build around it is:

Client → Workspace → standardized lifecycle templates → experiments → shared reporting layer

That last piece is important. I wouldn't expect the ESP itself to give you the ideal cross-client agency dashboard. I'd pull campaign data into a central BI layer (Looker Studio, Metabase, Power BI, etc.) and normalize metrics such as:

• activation rate
• conversion rate
• revenue/user
• retention
• unsubscribe rate
• incremental lift
• experiment winner
• time-to-conversion

When I'd choose Braze instead

If your clients are larger consumer apps with substantial event volumes and sophisticated cross-channel journeys, Braze is probably the stronger platform. Its Canvas supports complex event-driven journeys and Experiment Paths, while its reporting can compare campaigns and Canvases and analyze variants. www.braze.com

Braze is also particularly interesting for an agency now because its 2026 MCP functionality supports working across multiple Braze workspaces and querying/operating campaign data conversationally. docs.customer.iocustomer.iowww.braze.comwww.braze.com

So, in one sentence:

Customer.io for an agency that wants flexibility and efficient multi-client operations; Braze for an agency serving enterprise clients where sophisticated experimentation and cross-channel orchestration justify the complexity.

If you tell me what kinds of clients you manage (SaaS, ecommerce, apps, B2B) and roughly how many, I can narrow this down to the best 2–3 and compare multi-client management, templates, experimentation, reporting, API access, and pricing.

If you mean an agency that runs lifecycle/CRM automation for multiple client brands and wants to repeatedly test, benchmark, and report on those campaigns, my top pick is Customer.io.

My ranking

Why Customer.io: its workspaces are effectively separate environments, each with its own profiles, automations, metrics, and data—which maps nicely to an agency/client structure. You can also copy workflow actions between workspaces. docs.customer.io

For experimentation, it has A/B tests, multivariate testing, conversion goals, cohort analysis, and campaign-level comparisons, so you can actually operate a test-and-learn lifecycle program rather than just automate email blasts. customer.io

It also supports email, SMS, push, in-app, webhooks, etc., with visual branching and event/segment-based triggers. customer.io

Where I'd choose something else

Braze if your clients are substantial consumer brands and you're doing very sophisticated experimentation. Its Canvas supports experiments across entire journeys, different paths, channels, cadence, and control groups, with advanced optimization on top. www.braze.com

Iterable if you want an enterprise-grade cross-channel platform with particularly strong marketer-controlled experimentation and journey orchestration. iterable.com

ActiveCampaign if the clients are smaller and the agency cares more about cost + ease of implementation than sophisticated lifecycle experimentation.

One important distinction

If by "compare campaigns for several clients at once" you mean a single agency dashboard where you can see Client A vs. Client B's lifecycle experiments and KPIs side-by-side, that's different from simply having multiple client workspaces.

In that case, I'd probably build the stack as:

Customer.io/Braze → warehouse → agency-level BI dashboard

rather than trying to force the messaging platform itself to be the cross-client reporting layer.

If you tell me roughly how many clients (5, 20, 100+), B2B vs. B2C, and whether you need email only or email+SMS/push/in-app, I can narrow this down to the best 2–3 options and compare pricing, multi-client management, experimentation, reusable campaign templates, permissions, and reporting.