the kx100 is a big little bike !
To answer your question, the 125 mx bike is a rocket, which needs to be ridden in the power band - you know, where you get into the peak power, and hang on for dear life - if you have never experienced a two stroke power band , be prepared to wrap yourself around a tree, take off in a wheelie, high side, hit the garden fence - in short, wreck and pee yourself all in one breath. Ride it like your four stroke, and it will sputter, bog down, and not go very well.
At your height, and with your experience, going for a bike which you are going to enjoy riding is the way to go.
Best if you can get a ride on a few different bikes, see how you like the weight, the power delivery, the power band. You don't want to get hurt, so look at all your options.