Ubuntu has a wider selection of more up-to-date packages and lots of handy helpers (like tasksel and other config wizards) that will make your life a lot easier.
For anyone who suggests that Ubuntu is for desktops, not servers, remember that the Wikimedia Foundation runs Wikipedia exclusively on Ubuntu servers. [Citation: http://www.ubuntu.com/products/casestudies/wikimedia]
All said, there's not a huge difference between centos and ubuntu for basic web server administration - you still need to understand the underlying tools - apache, mysql, dns, etc. regardless of the OS chosen.
From my personal experience and from those two choices I would pick Slackware ;) If forced to pick something other then probably debootstrapped Debian.
go for debian stable releases, some of our servers have ran for over a year without rebooting/OS upgrade on Amazon EC2, or maintenance.
ubuntu is based on debian, so your transition will not be tough