The performance difference between a 750 and a 600 sport bike is a good bit. The difference between a 600 sport bike and a 500 budget bike is ALOT.

Going from a 750 to 600 you are going to notice mostly just a power difference. The brakes, suspension etc are all pretty good. The Ninja 500 is a budget bike with crappy brakes, suspension and power. If you are looking for simple transportation with alittle excitment they are ok. But if you like to have alot of fun and push the limits on open windy roads, the 600 sport bike would be perfect.