KDE and Gnome are GUI enviorments for linux. Both work well, and it's mostly a matter of preference. You should also realize there are other options, such as fluxbox, xfce, etc that are more lightweight.
It's all about personal preference here, I would get ubuntu live CD and Kubuntu live CD and try them both. I prefer gnome over KDE, but I prefer XFCE4 over gnome (really lightweight, and I don't use the GUI that often.