Please help! Motocross emergency!?

I need to know if I should get a Kx 125 or a Kx 250f ! I'm 5'6, 14 years old and I weigh 135! Not to brag but I can throw it around pretty good! I'm going to be racing motocross and arencacross! So please if you have any suggestions/ answers on what I should do please tell! Thanks for your time!