I would say, it depends on kind of dev team you are and your application needs.
For example if there are more querying required, it mostly means more work in Redis, where you can use different data structures to suit your queries. Same is easier in MongoDB. On other hand this is often extra work in Redis would most likely to pay off with sheer speed.
MongoDB offers simplicity, much smaller learning for guys with SQL experience. Whereas Redis offers non-traditional approach hence more learning but huge flexibility.
Eg. A cache layer can probably be better implemented in Redis, and as for a more schema-able data MongoDB might be better. [Note: mongodb is schemaless]
If you ask me my personal choice is Redis for most requirements.
Lastly, I hope by now you have seen http://antirez.com/post/MongoDB-and-Redis.html
