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if you you have some power the c6 will be better but the c6 causes more drivetrain loss than a c4. A mild small block, i would use the c4 built something more serious i would use the c6.
If you don't plan on pushing alot of power then use the c6 because you can sell the small block c6 for alot more money then the c4. Racers like the small block c6 because they can build a stout, high revving motor and have a transmission that can take the abuse better than a c4.