There is a nice site www.distrowatch.com with many usefull informations about Linux and some other free source OS's. There you can see Top-Pop list "Page Hit Ranking" with status for 100 distro's. Ubuntu is at the very first place because of its free CD's, not entirely because of his quality. Kubuntu is almoust the same distro, with KDE instead of GNOME, but at the moment NO free CD's and it is on 12-th place.

Is Ubuntu better than knoppix ??
They are diferent, imposible to compare.