The best resource for comparing the two is on the Couchbase.com site: http://www.couchbase.com/couchdb
In the end it really depends on what you're going to build. Apache CouchDB has replication and can host static web content for use with its HTTP/JSON API, Couchbase has memcached and multi-node horizontal scalability "out of the box."
If you have more questions, let me know. I'd be happy to answer them!