IMO, Stick with the KDX
the KDX 220 is a great bike. It's as easy to ride as any 4-stroke and it weighs substantially less. With a few easy modifications, it really rips. I can easily keep up with my buddy on his WR250F (which is one of the most aggressive four stroke trail bikes). I can also start my bike in one kick--every time--and reach the ground with my feet (I'm 6'0" and I can't flat foot a WR). If you're comparing one to an ordinary 4-stroke trail bike (XR, DRZ, etc.), you should compare it to a 400, not a 250. On top of all that, it's substantially less expensive than most four-strokes.
The only downsides are (i) you have to mix your fuel; and (ii) the suspension is outdated (if you're heavy, new fork springs are a must and, if you are really fast, you'll want the forks re-valved).
I have a KDX. Accordingly I am biased. Keep in mind, that I returned a WR250 in exchange for the KDX (and I don't regret the decision). Also, for what it's worth, I have an old 1982 KDX 175. It still starts on one kick and rides great (considering its vintage).
Whichever way you go, good luck and have fun!!!