If you are going to race get an 85 (CR85, YZ85, KX85, RM85, or KTM 85SX). If you think you might be growing a lot soon, maybe you should get a small 4-stroke trail bike (ttr125, klx125, drz125, ect.), ride it for a couple of months, and sell it to get a 125 2-stroke (YZ125, CR125, RM125 ect.). The small 4-stroke trail bike should have pretty good resale, and if you buy used, you wont lose much money.
I'll let someone else tell you about the 2-stroke and 4-stroke differences because I can't really tell you from personal experience.