Earlier quoted context omitted.
If you want to create offline-first apps, I would use Postgres as the main datastore and use Couch to sync data between client and server. You'd have to decide which of your data would live where (or if you wanted to use Couch as a way to transfer data from the server to the client). Couch is a very good datastore for that use case, though, so I would definitely use it in some capacity for your purpose.
Can you expand on this? Do you mean you'd use Postgres as your main store and periodically "flush" data from CouchDB to Postgres (probably as JSON)? I don't see how that would work reliably. On the client would you source CouchDB or Postgres? Presumably you'd access CouchDB directly, but then why even use Postgres (for that subset of data, anyway).
If it's data you're going to want to run analytics on and sync to the client, you're probably going to have to store it in both places, I think.