This is a topic that will generate alot of conjecture, and at the end of the day you will believe what you believe. On a MX track there is noway the 2t will run with the 4t, if track conditions are ideal they will be able to do a better job. Look at the 125 nationals a few years back, the ONLY person to make a 125 work was Bubba, When you looked on the starting gate at the Supercross or the nationals 95% of the bikes were 4t, why because if you didn't have one you were going to get your a$$ handed to you. I've had both and can honestly say there is now way a 125 can run with a 250f, Kawi 125's haven't been the fastest anyway, mine were heavily modified and an equal 4t rider would blow them away. Its not all about displacement either, its more about putting the power to the ground, the 4t puts it down. The analogy about the big block and the small block with the stall converter illistrates the point, the 4t puts it down the 2t is spinning the tire, 4t traction 2t roost=spin. Mind you I'm not a fan of the 4t, but if your going to race you have to have one to be competitive. Of course in a couple of years it isn't going to matter because there won't be any 2t MX bikes left.