Knoppix uses the K Desktop Environment, which is quite a lot like Windows; but Ubuntu uses the Gnome desktop environment which is much less like Windows.
Knoppix has support for MP3s and a couple of other proprietry formats out-of-the-box, whereas with Ubuntu you need to install these, which still isn't hard. Knoppix comes with more programs, but Ubuntu has a large number of programs available through repositories.
Also, Ubuntu has a more consistant user experience. I noticed that with Knoppix, some of the setup programs are terminal-based with the occasional GUI dialog box. With Ubuntu's setup apps, you get real GUI windows. (for some things you might need to go into the command-line; your milage may vary).
Ubuntu's installation procedure is supported, whereas Knoppix's isn't.
You'll probably be happy with Ubuntu, but if you don't like Gnome you can try Kubuntu (Ubuntu with KDE)