Depends on what you want to do with it. You're talking about the ZX-6R (636cc) versus the ZX-6RR (599cc), right? Because this year the 636 has been elmininated in favor of just the 600cc sized motor. I guess I'd say:
Street bike/Casual Track day: ZX-6R (636)
Track Bike: ZX-6RR
Race Bike: ZX-6RR chassis with a ZX-6R (636) motor stuffed in it
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