I have not compared the KX500 to a 450F but I have rode a YZ450F 07 model back to back with my YZ250 05 model and it's a world of differance between the two. I was able to ride faster and smoother on the YZ450F then my bike. Could be my style or some bike set up differance but I was blown away at how much my riding improved on the big four stroke. At the same time the guy that owned the 450F could get on my bike and just wax me on the track or in the woods. Rider skill and comfort level are key. 2-strokes are what I perfer but they do required more rider input. I feel if can you ride a 2-stroke at a high level then the four stroke is just going to take you a step higher. My son is only 9 years old and will contuine to ride 2-strokes (cheaper on me) until he out grows a 125 2-stroke. I think the 2-stroke make good training bike for riders due to the more input they require thus the rider learning more. Sorry I got off topic.
