I think it's not clear for you the differences between one and the other.
MongoDB is a nonstructured (at least, in definition) distributed document storage. For clearing purpose you can think about it as a high scalable and not so high performance JSON storage (though actually it uses BSON). It has a simple but effective multitype index system, replication and sharding, and a lot of more nice features.
Redis is a simplest key/value store, with transient data caching. It can works as an efficient queue system.
Looking for more specific information you will find a lot of scenarios where both them are used, Mongo as a true persistent canonical storage, and Redis as an transient/cache storage.