Just recently switched to Debian for a server distro. Early in my linux learning, I had been messing around with Gentoo, Fedora, and even Ubuntu. It's taken me so long to understand what Debian is and how important good package management is, but now I'm grateful. Thanks Debian!
Ubuntu's stated focus from the outset is making an enduser distro. Their success in mindshare made them popular in the server arena, but there are little things here and there where it falls behind. Making a server not based on an LTS version is one early trap to fall into (all debian versions are effectively LTS), but more importantly, debian is not fond of retrograde breakages. Ubuntu is much more of a moving target than debian - when you want to find out how to do something, it often changes between releases (one mildly common desktop task (which I can't recall at the moment :/ ) had five different ways to do it over the past five releases...). There's more unique stuff in ubuntu - like upstart, a core feature which no-one else uses - and that reduces your troubleshooting info pool. Ubuntu also has lots of automagic-helpery stuff, which is good for an end-user distro, but gets in the way a little on a server. One example is the 'did you mean?' thing when you make a shell typo. That consumes time and is pointless on a server.
I'm not particularly familiar with CentOS. I've been an apt user since rediscovering linux, and every time I venture onto an rpm-based system, yum just spews hard-to-read crap at me. It feels like rearranging deck chairs, so I head back to what I know.
