night and day
a 250 two stroke and a 450 4 stroke are gonna be fairly comprable in terms of peak power. the 4 strokes have better tractablility and low end torque. the 250s are gonna wind out quicker and hit harder, but they are getting alot closer with these 4 strokes. a 500 two stroke is completely different. dont worry about being too big. ive ridden and raced a 250 and im, well was 210 when i had my last 250. I ride a drz400 now. depending on where you ride unless its just wide open most of it is rider. If you get left behind its not because of your bike. every bike has its disadvantages and advantages. If your riding through the woods and your a good rider you could probobly kill those guys with a kx80 because they cant use all their power on the tight stuff and an 80 is very light and easy to handle. on the same note a 250 four stroke for tight stuff is great because its so light. yellow or green the rmz250 kx equivalent would be good. how much riding experience do you have? that will play a big part in bike choice.
matt