I'd buy any bike that fits your price range, the bikes really aren't a TON different between the KX, YZ, RM, and CRs. They are different, but everyone tunes thier bikes different. Look what is out there in your area, and what mods, the condition of the bike and go and ride it. You might beable to find a 1996 KX250 that actually rides better than a 01 YZ250 based on how it is tuned, your riding style, how many hours are on the bike, etc, etc.....(Stock for stock the YZ would be better, but buying a used bike is different than buying stock).
Good luck.