Isn't nginx more often than not used as a front controller for apache/tomcat/etc?
Benchmarks on my projects show that the difference is insignificant. Since I currently do not have memory constraints I am still using nginx + apache with mod_php.
But like I said, nginx is great for doing loads of things and I sort of "quit php" recently so I don't really care about php performance anyway.
Personally I use Nginx > PHP-FPM, so there is no other web server on the box.