Well to be honest you cant even compare the two. Centos is a Enterprise Linux OS. And Fedora is a developmental OS. Centos 4.0 will be supported until 2012, and Fedora Core 4 will be supported to the end of this year. Fedora is not a good choice for servers, its a fast pace development OS. There is a New Core every year, and it order to get full support you need to upgrade your server OS every year. With Centos 4.0 you could run your server until 2012 and never have to worry about upgrading the OS. I cant comment on Ubuntu, but if I were building a new server I would use Centos.
