aaaantoine said: Indeed.
I've used Gnome 2.24 and KDE 4.1 at length. Obviously at this current point in time Gnome is more complete than KDE because KDE just underwent a major transition step. The counter to that is that if you must, KDE 3.5 is still vaguely supported; otherwise, wait 3 weeks for 4.2 (or a few months if you want to stick to the Ubuntu repos).
Qt is GPL as opposed to GTK's LGPL, so you can't write proprietary software using KDE's primary toolkit unless you purchase a special license from Trolltech/Nokia. If that has any impact on your ideals one way or the other (maybe you like enforced open source, or maybe you like the freedom to issue your GUI based software under your choice of license), there you go.
So really, it's just a matter of preference. Try out both. See what you like. I've found a few features in KDE that I like over Gnome, but there are features that I miss from Gnome as well. Everything is a trade off.
It's called a dual license, it's not a "special" license, if you write a proprietary application who is going to support it?, pretty sure freesoftware devs wont and I think the customers of the app will want it fixing quick because they paid money for it(supported).