It's all about the set-up. I had a very well dialed in '91 KDX250 and my buddy had a well set up '92. In stock condition, the KDX250 needs some serious TLC (Kawasaki did not do a good job with supsension or jetting). After mods and set-up, I much preferred my KDX250 to my '90 KDX200 (also well dialed in) though in stock condition the 200 was probably better than the 250.
I have ridden well set up RMX 250's (susension revalve, jetting, pipes, steering dampers, etc...) of various years and they were very comparable to a dialed in KDX250. Perhaps SLIGHTLY better than the modded KDX250, but not by much. Stock to stock, the Suzuki is usually better. Bottom line is the KDX250 needs some work to get the most out of it. If you are not prepared to finish what the factory started, get another bike.