When it comes to creating a REST web service with 60+ API on akka http. How can I choose whether I should go with akka streams or akka actors? In his post, Jos shows two ways to create an API on akka http but he doesn't show when I should select one over the other.
This is a difficult question. Obviously, both approaches work. So to a certain degree it is a matter of taste/familiarity. So everything following now is just my personal opinion.
When possible, I prefer using akka-stream due to its more high-level nature and type safety. But whether this is a viable approach depends very much on the task of the REST API.
If your REST API is a service that e.g. answers questions based on external data (e.g. a currency exchange rate API), it is preferable to implement it using akka-stream.
Another example where akka-stream would be preferable would be some kind of database frontend where the task of the REST API is to parse query parameters, translate them into a DB query, execute the query and translate the result according to the content-type requested by the user. In both cases, the data flow maps easily to akka-stream primitives.
An example where using actors would be preferable might be if your API allows querying and updating a number of persistent actors on a cluster. In that case either a pure actor-based solution or a mixed solution (parsing query parameters and translating results using akka-stream, do the rest using actors) might be preferable.
Another example where an actor-based solution might be preferable would be if you have a REST API for long-running requests (e.g. websockets), and want to deploy the processing pipeline of the REST API itself on a cluster. I don't think something like this is currently possible at all using akka-stream.
So to summarize: look at the data flow of each API and see if it maps cleanly to the primitives offered by akka-stream. If this is the case, implement it using akka-stream. Otherwise, implement using actors or a mixed solution.