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. 

http://instagram-engineering.tumblr.com/post/13649370142/wha...

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.

In short - faster, easier on ram usage and easier to configure.