I played around with Ubuntu and Kubuntu for a while before discovering Debian proper. So good! The only downside for a desktop system is the slow release cycle (but that is also a strength). I personally run the 'testing' release to get new software reasonably quickly.
In my experience, so take this with a grain of salt, Debian owns (K)Ubuntu in stability and performance. I also like being able to install a base system and only add *exactly* what I see fit to.
Although now I come to think of it, perhaps the Ubuntu server install was the same sort of thing. Oh well. It'll still be brown. ;-)
One other thing, I run Gnome exclusively, so I can't comment on KDE on Debian.