I'd say it really depends on what you're storing and how you're using it.

Redis is great for situations where you need a very large volume of reads and writes, and have a fairly simple data model. See http://simonwillison.net/static/2010/redis-tutorial/ for more information and some use cases.

MongoDB allows for data to be structured and queried in more complex ways, and touts itself as more of an alternative to an RDBMS than Redis does. If you're looking to use it with Django, check out MongoEngine at http://github.com/hmarr/mongoengine