I was actually in the same boat when i got rid of my heavily modded 02 yz125 for an 08 yz450f just a few weeks ago and i will never look back. I am 6'0 180 lbs. and also ride about about %80 trails %20 track. The 450 is set up perfectly from stock for me and is a very very well handling bike, even through the tight trails, and has very smooth usable power. It has only been about three weeks and i am already more comfortable on my 450 than i was on my 125 and i had that bike for 6 years.