so i know this is over a year old but figured id throw my 2 cents in neways incase some one else is considering picking up a 450 sized sport quad..
the kfx450r is a damn good trail quad, it does very good on the track to. but it comes alive and owns on the trails due to it mainly being designed for trail riding over track riding. if you are doing a mix of trail and track riding with more trail time id go with the kfx. but if you are more of a track/racing person. hands down the yfz450r would be the way to go in my eye's. awsome thing is that both models for the 2010 line up use efi, so when you do exhaust and intake mods, or internal motor work. you dont have to mess with jets, just plug in a fuel controller and go for a ride.
if you want to compare specs a good place it do it at is the woodsfuncenter.com website.