most of the reasons for running ubuntu are dekstop oriented. Centos is probably a better bet for servers.
Ubuntu has great Long Term Support - which is exactly what this is about and the apt package management is far superior to Yum (IMO at least) - Plenty of high profile companies use Ubuntu for servers.
Debian has always been my favorite, and currently we use Ubuntu on servers as the OS-packages-as-shipped are more up-to-date; which turns out to be quite important in web-dev-land.
Frankly I dont know what CentOS/Redhat does better then Ubuntu nowadays, apart from selling enterprise stuff like JBoss :)