What's your current data size? How much do you think it would grow over a period of next 5 years?
NoSQL is not a panacea. It has it's own pros and cons. Can you forsake consistency over availability or vice-versa? Understand CAP guarantees (https://en.wikipedia.org/wiki/CAP_theorem) and the trade offs that you can make, before making the decision of migrating to NoSQL.
Ok, here's the comparison of Mongo and CouchDB.
Mongo:
- JSON based document store(BSON actually when it's stored in the file system!)
- Sharding, Journalling and Replication(Master/Slave) support
- Different storage engines(MMap, WireTiger etc.) for different use cases(Is your system write heavy or read heavy)
- 16MB is the max size limit per document, use GridFS if you're planning to store huge files
- Javascript based querying
- No joins or transactional support, you can still perform atomic operations
- Tuneable consistency through settings like Read Preference, Write Concern
- Text and geo-spatial search features
- It does support Map/Reduce type of queries
- CP system from CAP standpoint
CouchDB:
- Again, document store, but store the data in custom binary format
- Multi-master replication
- MVCC system, hence offers ACID semantics
- AP system from CAP standpoint
Hope this helps!