I vote Gevent because of its speed, small resource footprint, and its simple synchronous-looking API, but be aware of its limitations. The greenlets (threads) spawned by gevent run in a single OS thread, so you won't be able to take full advantage of all the cores on your server with Gevent alone. Also, Greenlets run cooperatively meaning each one gains control of the CPU until it's done with its computation or hits an IO call. This is great for IO bound computation, not so much for CPU bound work. Even with these limitations, I still think it'll work well with in your application.

Check out this comparison of Python servers: http://nichol.as/benchmark-of-python-web-servers