Why user exposure matters for Linux distributions, especially on desktops
Red Hat Enterprise Linux (or equivalently CentOS) has any number of things going for it from the perspective of sysadmins, but one of things it does not have is user visibility. Ubuntu is the hot Linux distribution these days, despite its issues, with special acclaim for its desktop experience.
This user visibility matters, contrary to what some people believe (or would like to be the case).
A good part of why we run Ubuntu on our core servers is that research groups were already running Ubuntu on their machines, both desktops and compute nodes, and they wanted to us to have the same environment, partly because it was what they were already familiar with, and partly because it meant that they could easily move programs back and forth between their machines and ours. Those research groups did not select Ubuntu because they had gone through a careful technical evaluation of which Linux distribution would be better; they used Ubuntu because it had the mindshare and because it worked well enough to justify its PR.
In a nutshell, that is why user visibility matters: these decisions do get driven from the bottom up, with users advocating for what they are already using and are familiar with.
(Also, it is easier to sell something to users if it already has the visibility with them. I am sure that there would have been people asking why we weren't using Ubuntu if we'd made a different choice, and yes, their opinions matter.)
I say that user visibility especially matters on desktops because desktops are the easiest and the best place for users to get hooked on something. They're the easiest because in practice they're the machines that users have the most control over, and they're the best because they're what users use all the time.
Why Ubuntu's LTS releases are inferior to Red Hat Enterprise Linux
It's time to update my view of Ubuntu with my most recent set of feelings. Well, with why I feel my most recent set of feelings, which is that Ubuntu LTS is significantly inferior to Red Hat Enterprise Linux.
Ubuntu's LTS releases (Ubuntu 6.06 and Ubuntu 8.04) promise five years of support (hence the 'Long Term Support' label). This support is why we're able to consider them, since we need more than 18 months of support that you get with regular Ubuntu releases; we're simply not in a position to update our servers that frequently.
(There are two reasons. First, moving operating systems in a production environment requires a fairly large amount of careful testing (and a certain amount of dealing with changes). Second, we run login servers and our users do not want to have to do migration work that frequently either; they have better things to do with their time, like complete their PhDs or do research.)
The problem is that in practice Ubuntu's 'long term support' is actually only 'long term security fixes'. I have almost never seen Ubuntu fix a problem that was not a security problem, even when the problems are reported in Ubuntu's bug report system (and in one case, even when the problem let an ordinary user crash the kernel). The inevitable result is that we have an ever-growing catalog of bugs in 6.06 that will never be fixed.
(I think that Ubuntu does fix bugs under some limited circumstances; what they really don't seem to do is fix bugs when the fix would require backporting things into the old 6.06 version of packages.)
By contrast, something like Red Hat Enterprise Linux does provide real long term support, where even non-security bugs will be fixed (at least for a while). This is not just theoretical, in that I have seen actual RHEL packages released to backport fixes for mere bugs.
(I am also relatively certain that Red Hat would consider 'user can crash the kernel' to be a security bug.)
Ubuntu, LTS releases included, still has an unmatched selection of packages (and is what users have heard of, which matters more than you might think). But there is less and less enthusiasm here for running it on 'backend' machines, machines that users don't log in to or run programs on, and I can't say that we're very enthused about it even on the login servers.