Why not have Both?

(I couldn't resist).

Actually your question is a little weighted.... Debian is an Operating System... KDE is simply a window manager...

Debian allows access to all the software in the debian repositories... KDE on Maemo doesnt really add access to any extra apps except the KDE ones that Penguinbait ported...

Debian runs both Gnome..And KDE... along with IceWM; Window Maker; XFCE; and many others...

Hard to compare apples to oranges...