Are you road racing or drag racing?
The unsprung weight is more critical in the road racing. For drag racing, any weight is important. Also, the 9" has a greater pinion angle which means more frictional loss.
I'd say, that unless you're pushing some BIG power & running slicks, a well built 12-bolt should be fine.
Gordon Arnold, 2000 WS6 T/A, A4 converted to M6, RAM PowerGrip HD, PPC Long Tube Headers, Random Tech Cat-Back, PPC Suspension, HP Motorsport Torque Arm, SLP connectors, Bilstein sport springs & shocks, PPC smooth bellows, PPC wires, MTI lid, HPP3 Daily driver & hobby road racer. .... etc... (and a moderate 230/230 110 LSA bump stick waiting to go in).