My understanding is that Couchbase is just a key-value store, while RethinkDB has much more features including complex queries, sorting, joins, indices...
Couchbase conflict resolution is really basic, assuming you mean distributed in a wan context (eg xdcr). Riak would represent the state of the art in this respect (random internet endorsement, I'm not affiliated with anything, just a user who has worked with both systems).
Our XDCR doesn't have quite the power of our mobile sync conflict management, but I'd wager to say what we do for mobile is unbeatable as far as delivering unsurprising behavior for offline / p2p applications.
