Whose choice? Not mine... not that I have anything in particular
against Ubuntu (in fact, I'm on an Ubuntu system as I write this, because it's our corporate standard desktop here), but I still prefer Red Hat based distros, whether it's RHEL, CentOS, Fedora or what-have-you?
"Why?" you might ask. Well, TBH, a lot of it is just familiarity - I've been using RH based distros since the Red Hat 5.2 days, so it's what I know already. But more to the point, it just works. I haven't felt any pain using Fedora, CentOS and their ilk, that has ever compelled me to go looking for a different solution. And that's even more true for servers, where I don't care about prepackaged video codecs or sound or anything.
The one big thing that everyone has always touted as the edge that Debian/Ubuntu have over RH systems, is apt. But after having used Ubuntu for 2 years now, I still haven't found any regard in which is apt is particularly better than yum. Yeah, yum used to be dog slow, but that hasn't been a problem in ages. And I don't know about you, but it still annoys me that I need one command, yum, to both search for packages and install them on CentOS/RHEL/Fedora, but I need apt-cache and apt-get to do the same thing on Ubuntu.
Anyway, props to the Ubuntu folks for the release. I do lean towards Red Hat derived distros, but I won't say that Ubuntu is bad or anything.