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...