raptor -
a couple things you oughtta know
1) a 125 is no more forgiving if you accidently blip the throttle than a 250.
2) on an MX track, an accidental blip of the throttle is no biggie because you have about a 20 foot wide "trail" in front of you.
3) chances are you are not skilled enuf to ride a 125 at the edge of its potential, either.
4) while the 250s(2 stroke) are indeed substantially more powerful than the 125s, they have a wider useable RPM range, which makes them easier (IMO) to manage.
5) your reasoning about the 250f is sound.
6) for the uses you describe (trails, sand pits, etc) an MX 125 is a horrible choice, though at 150lbs you might be light enough to get away with it.
7) if you don't let off the throttle on the face of a jump, engine braking is not a problem on a 4 stroke.
8) bottom line, i think you'd be very happy with a 250f, but i think you'd like a 250 2 stroke almost as well.
jeremiah