They're basically the same. By default Kubuntu uses KDE as mentioned. It's probably going to be a more comfortable transition for a windoze user. However, Gnome seems to have more tools by default. They can be installed on Kubuntu as can the inverse (KDE tools/GUI on Ubuntu). They use the exact same repositories for updates/upgrades, so the code base is the same. They follow the same upgrade path, so either is acceptable. I prefer Kubuntu simply because, I came from windoze, switched to Mandrake and really like KDE.
I have used Red Hat, Fedora, Ubuntu, and several others who default to Gnome as the window manager. They're completely user friendly, but I just like KDE better.
Good luck and welcome to the Linux world. We're always here to help (well most of us...)
Mark