I remember reading from Instagram blog that they found Gunicorn to be less CPU-intensive:
We use http://gunicorn.org/ as our WSGI server;
we used to use mod_wsgi and Apache, but found
Gunicorn was much easier to configure, and less
CPU-intensive.
Nginx+n performs pretty well in general against Apache, and in my experience nginx as a front to something like uWSGI for Django is very easy to set up and use. There was a comparison of the various Nginx+n stacks, I think Nginx+uWSGI won out, with Gunicorn in a close second.
Gunicorn generally takes up less ram and less CPU than Apache + mod_wsgi. I've also had some major mem leaks with apache that have never occurred while using Gunicorn, I believe it restarts a thread when it gets out of control.