I would think the 600 would have more pull throughout the rev range and a higher top speed and with the right gearing be capable of aproaching 100 mph. The 500 will probably do about 90 and if I were contemplating long distance riding at speeds over 70 mph the 500 would get me there much faster and in greater comfort due to its smoother motor. If you cant achieve this performance then either your fetling or riding skills are not up to snuff. Sometime this year I will get the opportunity to test both bikes and will let you know how it goes,