It really comes down to personal taste.
I currently have two PCs with KDE and one with GNOME, and I have to say I prefer KDE. My first Linux experience was using GNOME. I began using KDE five or six months later when I started using SL, although I had dabbled a little with KDE under Kororaa some months before, so I was already aware of KDE. Initially I thought that GNOME looked 'cleaner' and minimalist in comparison to KDE, and I also felt that KDE had a sort of kiddy look to it, what with the bouncing icon when you launch an application via the GUI, the cute KDE dragon, some rather twee icons and so on. I wasn't sure which one I preferred until very recently, and finally concluded that I prefer KDE. I find it somehow more ergonomic than GNOME and I personally prefer the look of the KDE desktop and icons to those of the more-austere GNOME desktop, although there are plenty who would say the exact opposite.
Mind you, I prefer some of the GNOME applets and applications to their KDE equivalents (e.g. GNOME Volume Control to KMix, GNOME Print Manager to KDE Print Manager) but then it goes the other way too: e.g. I prefer Konqueror to Nautilus. But then you can use any of these apps under either desktop, so it's no big deal.
They both come on the SL LiveDVD and you can install them both to your HDD, so the best thing to do is to try them both over a period of time and see which you prefer.
Coincidentally, yesterday I read the following two-part article in Datamation, comparing KDE and GNOME:http://itmanagement.earthweb.com/osrc/a ... hp/3671906