I’ve used them both in the past, but prefer Xfce now. So I’m probably not too biased either way on Gnome v. KDE. I’d say they’re both extremely well-supported, popular, respectable, and safe choices. They’re quite different in style though, so odds are you might find you have a preference for one or the other. Go with whichever you like best.
Sylpheed is the best. I thought everyone knew this.