Personally I'd hold out for the R. Better aftermarket, faster, HONDA reliability. I don't believe the motor has that much in common with the KX250, IIRC. But I could be wrong. Get a TRX-R, and you'll love yourself. My bud had one that was stock, and his old man has one with a 310 kit. Those things were sweet, and FAST. He's running a banshee now, now that's speed! So in conclusion: Wait for an R, unless those things are so cherry you'll never find another like it.
Allan