mongoengine

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

ive been using mongo and tornado together for a while, and its great. i initially chose mongo over couch because of its fast ad hoc querying. the python client is also well-written and well-documented. one thing, though, to watch out for is that only the "unstable" dev version of mongodb (1.3.x) has read concurrency - before 1.3.x, mongo uses a global read/write lock per operation. general and index-assisted reads are ultra-fast in mongo, but a bigger map/reduce or group call will block other requests until complete, possibly causing traffic to back up. because of that global lock, all writes block, too, but i've never had a problem with that IRL. writes are super-fast. if you want an orm for mongo, check out mongoengine on github: http://github.com/hmarr/mongoengine/tree/master/mongoengine/