I can obviously only give the Realm perspective. Realm and CouchBase are fundamentally different types of databases, in that Realm is an Object Database (allowing you to work with complex graphs of interconnected objects) where CouchBase is a Document Database (working with objects that are hierarchically arranged). Realm is also a first-class native database, where most CouchBase clients seem to be wrappers on top of sqlite or key-value stores, with all the performance limitations that entails (most benchmarks have shown Realm to be several times faster than Couch, but since all benchmarks are biased, you should obviously do your own before making any decision).
Functionality wise, Realm is also very different, with support for multi-threading, data binding, advanced queries and much more. There are also fundamental differences in how synchronization is handled, with Realm focusing on realtime synchronization with deterministic conflict resolution.
But in any case I would recommend you to try to build a small example project with each product and see which one gives you the best experience.