The 450 is the better choice. I have rode both the 450 and the 700, and right now I'm looking to trade my dirtbike for a yfz450. It's low to the ground, very light(only 350lbs. dry weight), and it has good power. Also, if you add some aftermarket parts, it'll have no problem keeping up with a raptor 700.
Hey. In my opinion, it's a lot easier to learn how to ride on a 4 stroke. But on the other hand, I learned on a 2 stroke and when I got my 09' yz250f, I felt a lot more comfortable on it. But for your size, I would say get the yz125. Good luck.