Couch Base and MongoDB are both Document Based NoSQL Database but they have some differences:
1.Data Model: For Data Modelling MongoDB used BSON which is a format very simmilar to JSON while CouchBase purely includes a JSON structure.
2. Concurrency: Couch base server is both optimistic as well as pessimistic locking while the MongoDB server is also of both optimistic as well as pessimistic locking but with an optional store machine called as the WiredTiger.
3. The Couchbase server has the capacity of storing binary values till about 20 MB, but the MongoDB server has got the ultimate capacity of storing huge files into a huge number of documents.
You can read this blog post to learn more about the differences between CouchBase and MongoDB.