the current crop of liter bikes are all fast, handle well, and have way more power then you really need to have fun. With that said I would reccommend the GSXR for its familiarity. when I ride my supermoto or my moms 636 for a couple days. Then hop on my busa, it takes some time to get reacquainted . When I had my GSXR 600 trackbike, I could ride around the track as fast as I could go for the weekend. Then hop on the busa and be right at home. The ergos are about the same. Sure the 600 had slighty more racey feel, but going back and forth didnt fill awkward or out of place.