I'm 5' 5" and 165 lbs and wanted a small, light, fun, flickable bike to play-race when I'm not riding my KDX200. I considered the KX100 really hard, but ended up with a KX125 instead. Really glad I did. My KX fits me perfectly and I can ride it at speed on the track with no squirrely handling. It has real suspension that can take anything that I can dish out. Had I gotten a KX100, I'd be dealing with a much smaller wheelbase and I'd have to beef up the supension. I play race with everything on the track, including a kid on a KX100, and the 125 really fits the bill. And finally, I got a 2-year old '01 KX125, purchased new in crate in '03, with Pro Taper bars, Pro Circuit pipe and silencer, PR2 suspension, VFII reed cage, new tires, new chain/sprocket for $2000, not much more than what a good used KX100 would've cost. And the 125 was setup for my weight, which a KX100 almost certainly would not.
My advice is to look around at used 125's.