I have both a 250f and 125, and was on a yz144 the other day (EG top notch job!). It was amazing I thought, had that lovely extra bit of power, so I was never clutching at all, and it wheelied like crazy. It was just so much fun and abit scary even! I'm still faster on the 250F, and it is easier to ride quick with all the low end grunt of a 4T. But if you stick with the 2strokes then you should consider a 144, the 19cc does make a big difference.
I'll admit that 144 was tuned for top end, so it was almost scary to ride, and if that 144 was anything to go by then I will never ever get a 250 2t, it would be overkill for anyone bar a pro.