Just my opinion. I'd probably lean toward the newer bike with the inverted fork over the '99 with the magnum 45's. They're just less maintenance. The Zokes need attention more often to keep the oil clean and the seals from leaking. This is a minor snivel, and I would'nt spend a bunch more money just on that cause. Both machines are quite capable.
On the KDX, Excellent bike. My brother in law is sold on them, and has owned 3 since 1996! He wears one out, and just replaces it. He even rides them stock, muffler, pipe, everything. He is a very talented rider, and places well in the expert lightweight class (0-200 cc), he consistently gets Gold medals in ISDE type enduros. Needless to say, it's rider not bike, and if you are any sort of trials rider at all, you'll do great on any bike. The Gas Gas will just be an easier platform to explore the limits of your abilities on. You'll be able to outride the KDX (which can be lots of fun).