I would suggested you go with NodeJS, if there is no personal pref to python. I like Python a lot, but for async I choose Tornado over node, and later had to struggle finding way to do a thing, or libraries with async support (like Cassandra has async in tests, but nowhere could I find way to use cqlengine with async. Had to choose Mongo since I already surpassed the deadline).
In terms of performance and async, Node far better than tornado.
