I am 15 6'0 170 lbs, and switching from atv to dirt bikes, i have ridden both all my life so i have good experience, i am looking to go into GNCC type racing. So should i go with 2 or 4 stroke, and what size exactly should i start out with? 250? i really dont care what brand, just one that has best performance compared to other ones in its class. :ride: