Ubuntu and Kubuntu are the same operating system, however, they are both using different desktop management environments. Ubuntu (being the original main distribution uses Gnome, where as Kubuntu uses KDE (however it's still Ubuntu. K = KDE). And naturally both of them operating in some minor different ways with file handling etc.
I found, from my experience, that most Uni's or companies with some form of knowledge of Linux distributions within the UK use Debian, Sun Solaris or SUSE (with the servers I've played around with, the majority was more Debian than anything else).
Standard workstations, however, most operate either Ubuntu (what with the vast support that the community, developers and such have to offer) or the older, now discontinued in support and updates, Redhat.
My preference is, or rather was, Gentoo. haven't used it in quite a few years now however back when I did use it that was one of the bigger most challenging distro's to install (no click and install features, all installed via code provided in a 300 odd page manual; yet it's changed a lot since then).
At this point in time I'd now use Ubuntu. Purely because I can't be bothered to go through the hastle of driver installations and what not (Ubuntu comes with a lot of driver packages pre-provided).
As for US universities, I haven't a clue. While I know that was the main specific question, thought I'd add my experience.