They serve different purposes. Riak is a DHT (for key sharding) layer intended to run on top of different backends, just like Dynamo. CouchDB has an integrated storage engine. Riak doesn't support range queries (nature of DHT). I'm pretty sure CouchDB does, which is easy for a single node. CouchDB doesn't scale in the sense that it doesn't do any automatic sharding yet. It's quite possible to use CouchDB as a backend of Riak, though it implies crippling some interesting features in CouchDB.

Technically, I found CouchDB more interesting as it's more "different" from other approaches.