Canonical (Ununtu) and Redhat (RHEL/Centos) are both rock solid distros. It's sort of a game of leap frog right now which one is 'better'.
IMHO, Ubuntu did a really good job releasing a well integrated version of their 'Long Term Support' release train with 14.04lts while Redhat kinda blew it with RHEL/Centos 7 because they didn't get some outside repositories integrated right.
For now, in general, Ubuntu 14.04 is ahead...this is likely to change in the future.
From my perspective as a server über n00b, I like using Ubuntu simply because of how popular it is—if I encounter a problem, it is easy to find a trouble shooting guide or a relevant stack exchange question, which hasn't necessarily been true with other distros.
I used to share that opinion but have to rethink that one since all the politics with Debian.
With all distros going Systemd, atleast for now, the difference will become even smaller. RHEL generally gets a bit more enterprise love while you are nowhere without extra repositories, so much for your stability.
Debian is trying their own 'LTS' releases starting with Wheezy?
A distro that got me a bit more excited over the last couple of months is Suse with Btrfs, Snapper, ...