Hi Mike,
Welcome to KTMforums.
If it were me, and I was a high end beginner, or low end intermediate rider,
I'd probably go for the 250 power wise. You are pretty light, and I have
ridden a 300 (my buddy's) , and it was real scary fast (I am 6'2" 210lbs).
I don't think there is a great weight difference, just power/torque.
I know when I went from my KDX 450 to the IT 465 (too many years ago

),
the power difference almost cost me a few broken bones.
Granted I am only talking 20 cc's or so, but it hit hard off the line.
I'd fish tail off of a corner, and just about go off trail.
That's when I went to 4 strokes.
What's the cost difference?
Regards,
Marco