Motomad-
It's rather hard to say, "Oh yah, you need this blah-blah-blah bike" without knowing what works best for YOU. If you were like me, I'd say "Go get a RM yadda-yadda-yadda." ITYKWIM.
There is another couple of questions that you need to answer. First, honestly, how fast do you think you're going to be? Not trying to a wise-ankle, but that is a very important self-evaluation you need to make. The KDX, simply by chassis design, is limited in its potential. While it has potential from stock, it has limits which fall below the other bikes. But, if you're only so fast and happy with that, then it doesn't matter that you cannot slide up on the bike to stuff the corners hard, nor will it bother you that the KDX becomes very scarey if shoved hard into corners, and so on.
Second question, is a smaller bike for you? Both the KTM200EXC, (I've taken a couple of short rides aboard one) and KDX are smaller framed bikes, at least they "feel" smaller than the GG200 and 250 everythings. (smaller compared to some of the 125's as well) If you're a taller + bigger guy, then maybe you might consider moving on to a larger framed bike, which would be NEITHER the KDX or the 200EXC. Again, my own experiences with the KDX and RM have shown me that I clearly prefer the longer size of the 250.
A couple of side notes; the KTM does vibrate, I rode a KTM with FMF exhaust and it ripped pretty good. The KDX can be improved a LOT with mods, but it does turn into a green science project. A 250 anything (except those 250F's, forget them) will shred any of the 200s.
Back to the KDX/EXC question... Here's what I see at the races- C class = plenty of KDX. B Class = a couple of KDX A Class = about nobody rides a KDX. One B class bud of mine, rides a totally stock 220 (except for lighter shock spring; he's 150#, 5-7ish) and he LOVES the bike.
I hope this helps. Good luck with the decision. :)