Of C, A & P (Consistency, Availability & Partition tolerance) which 2 are more important to you? Quick reference, the Visual Guide To NoSQL Systems
A blog post, Cassandra vs MongoDB vs CouchDB vs Redis vs Riak vs HBase vs Membase vs Neo4j comparison has 'Best used' scenarios for each NoSQL database compared. Quoting the link,
A recent (Feb 2012) and more comprehensive comparison by Riyad Kalla,
A blog post (Oct 2011) by someone who tried both, A MongoDB Guy Learns CouchDB commented on the CouchDB's paging being not as useful.
A dated (Jun 2009) benchmark by Kristina Chodorow (part of team behind MongoDB),
I'd go for MongoDB.
Hope it helps.