Honestly, I'm an OS whore. If my workstation was a town, and operating systems were citizens, I'd be the town bicycle. Everybody's had a ride. Hence what I'd suggest is making a bunch of small test installs of the various options, and seeing which best suits you.
Right now I'd say Ubuntu Server, but that's for ME. Remember, more than two thirds of major *nix distros are now based off of Ubuntu, (Mint, Arch, etc) which itself is based on Debian. Ubuntu basically IS Debian, just with more recent packages and more people now using it.
But if you're already comfortable with CentOS, that might be your best choice.
I'd say try them. VirtualBox is free, the install files for Linux flavors is free, so... make a bunch of little 60gb or so Virtual Disks on your main machine and try them. If nothing else it helps to be familiar with the differences between them, since not all *nix were created equal; even when they're all using the same Linux kernel.
Could even have some fun and try some non--Linux flavors. FreeBSD, OpenSolaris, Haiku...
