Are MongoDB and CouchDB perfect substitutes?
I've actually used both pretty extensively, both for very different projects.
I'd say they are equally well suited for the requirements you list, however there are quite a lot of differences between the two. IMO the biggest is their query-ability. CouchDB doesn't have 'queries' in the RDBMS sense (select * from ...) but instead uses 'views' which are more like stored procedures (essentially, static queries defined in the database (1)). MongoDB has much more 'usual' querying.
Essentially it comes down to your application requirements. If you give more information I might be able to shed some more light on what might matter in that situation.
(1): you can have temporarily, non-static queries in CouchDB but they aren't recommended for production use
Mongo uses more "traditional" queries. You turn on indexing on a per-key basis and use a SQLish query syntax.
CouchDB's views can do much deeper indexing and relationships but require you to do a little more work and understand the way the key sorting works for doing the queries.
There is a big difference in the replication systems as well. Mongo's replication looks a lot like most RDBMS solutions with masters and slaves and all that. CouchDB's replication is more peer to peer, no master/slave, every CouchDB is a node.