I went from a KDX220 to an '01 WR250F and was very pleased with the upgrade.
The WR is a great bike which is very reliable. Apart from a rare crank breakage (saying it quietly makes it sound not so bad), the bike has performed flawlessly for over 3,000 klicks of dirt riding.
Once the free mods are done (retard exhaust cam one tooth, cut grey wire, remove airbox lid, rejet), the bike will match the YZ250F for power (read: smoke the KDX220) and slice through the single track as good as any KDX.