The KX100 is a good bike for your size but not for much longer. So 2 key factors are how fast you're growing and what class you plan to race in, since you said you plan to race. As was pointed out, the KX125 is a full sized 2 stroke bike and may have to race against 250F's, which would be tough on a 125 (if so, AMA amateur rules now allow that 125 to be punched out to a 153cc). The KX100 is only legal in the supermini class, which is designed for kids moving up from 85's to 125's, but who aren't quite ready for a full sized bike. So if your local series has a supermini class you're good, if not, you'd have to ride it in the 125 class.
Have you tried sitting on either of these to see how comfortable you are?