I have been going back and forth over which Python API to use when interacting with Mongo. I did a quick survey of the landscape and identified three leading candidates.
Both MongoEngine and Ming depend on PyMongo - they just put some nice functionality on top of it. I'd recommend at least starting w/ PyMongo directly - that way if you decide to use one of the other tools and run into issues it will be easy to understand what is going on "under the hood". That said, I'm highly biased ;).