That totally depends on what you want to do with Redis/MongoDB. Do you intend to use it as your primary data store? AFAIK One of MongoDB's goals is to be useful

That totally depends on what you want to do with Redis/MongoDB. Do you intend to use it as your primary data store? AFAIK One of MongoDB's goals is to be useful for a lot of things you'd normally use a relational database for. Redis on the other hand has a lot of nice things to handle more specialized cases of data.