THe only disadvantage of an air cooled motor is that it doesn't dissapate heat as efficiently as a water cooled motor. At the end of a hard ride you may suffer a little more horsepower loss than the watercooled counterpart. The advantage is a much more simplistic design that lessens the chance event of a failure. IMHO, the kdx would me much more fun than an xr, but you will probably have to address the lackluster suspension. Easy enough though, as a lot of kdxers have bolted kx front ends on their steeds with very pleasant results. Just do the search thing and good luck.