What ever fits you the best and what type of riding you mostly do , the k5/k6 have had problems with the clutch and bad welds on the frame around the head stock go to gixxer .com lots of info there , the slipper clutch off the k5/6 will fit into a k3/4 and a k2 i think with a bit of grinding on the inside of the clutch cover for the adj,bolt , if the k5/6 fits you buy one but if your really tall and are'nt that sporty then a k3 , i own one and let me tell you get it set up and you will not need to go any faster , unless your on medication ,all depends on the type of riding , and we all know how long and how fast you can go on the street .saw a really nice k3 in here a month ago lots of stuff on it for a good price , why pay double for a new stock bike .