I recommend you request a demo of both products from Adobe. They're similar in certain aspects but quite different in others. Having a demo will help you a lot.
Here are few based on recent experience
ACC
- Can be deployed full cloud or hybrid. Hybrid is the common implementation
- Access to the Adobe campaign (CM) system tables in the hybrid deployment (believe me this is very helpful)
- More customizable.
- RT events (trigger messages) has no access to the recipient table.
- Supports inbound and outbound APIs
- Nicer interface but has a lot of features/options (maybe overwhelming for some)
- Thin-client based
- More admin work from maintenance, less ETL development since the admin is mapping existing DB tables as new data schema,
- Performance is as good as your DB server (hybrid installation)
- Both single-threaded when workflows run
- Similar limitations exist in both
- Limited new features in future roadmaps (focus in the future more on ACS)
ACS
- Full cloud deployment only.
- Don't have access to the Adobe campaign (CM) system tables
- Less customizable.
- RT events (trigger messages) has access to the recipient table, can reference recipient attributes for message personalization
- Supports inbound APIs only.
- Fewer options in the interface (Good/Bad)
- Browser-based
- Less admin work from maintenance, More ETL development since the admin is mapping & building the technical workflows to load the data from different data sources.
- More new features in future roadmaps
Thanks.