i'd say whatever you get make sure its a bit older and used... personally i'd rather see most people start out on a 1998 gsx-r 750 or zx7 instead of a 2005 zx6 or gsxr 600 or whatever, its too much money to have tied up in your first bike and the new 600's will eat up an older 750

and even though 600's an d 750's are different, its really just an arbitrary number, the most important thing is that you take your time and respect the bike and realize it owns you