I haven't tried XUbuntu so I have no idea what that is, or rather what the difference is. Now out of Ubuntu and KUbuntu, the differences are the desktop environment. Ubtuntu runs Gnome, KUbuntu runs KDE. I personally hate how candyfied & icon / appearance based KDE is (im a keep it simple minimalist), so I'd suggest going with Ubtuntu with Gnome as it's, to me, a lot more suitable. Take a look at screenshots of desktops that have KDE and Gnome desktop environments, see which appearance you prefer and decide accordingly . Although you can use either and install a desktop environment of your choice on top of the standard one.
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Just looked into XUbuntu, and it uses the Xfce Desktop environment with either Gnome or KDE, so it's more ideal for old low end spec computers. So if you have a half decent machine, consider my advice above.
I hate KDE, so personal preference on my part goes with Ubuntu. Gnome is a lot nicer. Again just my opinion. AMybe i just ahd bad experiences with KDE and good ones with gnome.... meh
I totally agree that Gnome is a lot nicer, but It's down to personal preference I guess. Although I'm sure there would be a skin out for Gnome that would make things appear like OS X. You can set the icons yourself, create fancy toolbars to look like OS X etc. So I still suggest Ubuntu.