What Rob said! The KDX is a great trail bike, and does an excellent job at what it is good at. Unfortunately, the KDX's time as a racebike was several years ago, and today, it is simply no match for a 250 MX bike, provided that the 250 has a flywheel weight and revalved suspension. I hung out a LOT of time & coin to make a 99 220 about as good as could be expected, but yet, as soon as I switched to a RM250, it was like shifting up a full gear and grabbing some throttle. HUGE difference, IF you are a faster rider.
Slow and medium speed guys will want to note that a 250 MX bike is not for the faint of heart at times, but overall, if you are a fairly good rider, the strong points of a 250's suspension and handling will far exceed the short comings. A 250's short comings include- harder power delivery, smaller tank, no lights. The lack of odometer & side stand are actually GOOD things, as the KDX odometer is forever wrong, and the sidestand only weighs lbs & gets caught on stuff. Plus, those WatchDog computers are TOTALLY cool, and the lack of one is an excellent reason to pony up. ;)
Also, some other very good options would be to consider 250EXC and GasGas 250.
Good luck with the decision!