Don't go with Apache unless you need to have a control panel, but I'm assuming that you don't need one. CentOS is mainly for control panels too. Both of these aren't exactly, optimized, in my opinion.
I would recommend using lighttpd (or litespeed if you have the money) on Ubuntu if you want to save money on the server hardware. In fact, I would say that you should keep the current configuration, but just migrate to a different OS. Maybe you could poke memcached into it to get the most out of everything.
It would only be logical to use Apache as the webserver if your servers can handle a much higher load, as Apache does have more compatibility and stability in some or most ways... but I agree that it has horrible performance when compared to a server with the same hardware running lighttpd or any of the other known light webservers.