Couch and Mongo are optimized for different use cases.

CouchDB is pretty much the only (open-source) game in town if you care about offline replication. It is also designed for extreme reliability and concurrency. CouchDB's programming model is designed to scale from a smartphone to a datacenter, and ops teams can scale it strictly in the HTTP domain.

MongoDB trades robustness and concurrency for faster serial performance for single clients. MongoDB's programming model is closer to MySQL or redis, so it isn't as big a leap for developers used to traditional 3-tier architectures.

I expect to see a lot of apps move between the two platforms as people start to find the sweet spot that they are interested in.