I have very little experience with motorcycles, but I can't imagine buying a 1000 over a 600 unless I was traveling over 100mph on a regular basis. The shorter gearing of a 600 might sometimes be a downside, but I think you'd have just as much fun as you would with a 1000 but without the extra weight and, according to those with more knowledge, risk.

They also tend to sound better (imo) and rev higher, revs = sex (don't ask what that means)