what kind of riding you wanting to do. Honestly if you're not going to be on a fast motocross track a 4-stroke would be your best option, especially for putting around in trails. It really is a matter of personal preference though. Just remember that a 4-stroke has a nice linear powerband although they seem to flatten out up top to me. A 2-stroke is pretty particular in its powerband. You either have next to nothing for power or you are full blasting like you just pushed a nitrous button.

Also take into account that a 125 2-stroke makes roughly the same power as a 250 4-stroke (although you would never guess it as the 250 4-strokes have got some damn nuts!) and the 250 2-stroke is equiv. to a 450 4-stroke.

Honeslty, for your wieght class I would either go 250 2-stroke or 450 4-stroke. 125's are out of the question as I am only 155lbs and a 125 is just enough power to keep me with a grin on my face.

All the above are recommendations for the motocross style bikes like the CR,CRF,YZ,YZF etc. If your looking into XR's and specifically enduro or trail only bikes, they are more comfortable, come in bigger cc's (like a 600cc), and are more comfortable. Post up what your looking for a bike to do exactly and I'll try to find some article reviews on different style bikes.