Then the 250 is the way to go, it will last as long as you take care of it and replace the top end like you should and run a smart mix ratio, don't take chances, mix 32:1 because a racing engine needs lube. However, their may not be many d class riders on 250's if you are seriouse about racing, most of the time the track is full of 125's in the d classes. You can grow into the 250 and still have more power than you'll ever need with it but don't be fooled, the 125 can tote a load as well and is more nimble than it's heavier brother. You can always change springs on the 125 to accomodate increases in your weight.