The lighter flywheel would equall less rotational energy which has pros and cons:
Pros: Less weight to turn, like underdrives, the engine would rev easier, less energy to get it moving. Less stress on the trany when hard shifting.
Cons: Less rotational mass won't have as much momentum which may hurt you launch. I won't have as much punch between gears.
I would go aluminum because mine has no low end tq problems and I do nothing but spin if I go any higher than idle. If your car was set up for drag on slicks, where the launch is everything, the heavy steel one would probally suit you best.
Think of this. How fast is 6000rpm's, I'll tell you it's, it is really fast. Now take a 30-40lbs disc and try too spin it from 800 to 6000rpm's, that is going to take alot of energy, that is how much energy that could go to your wheels, BUT on the other hand, now that you have this disc going at thin ungodly speed, how much energy will it take to slow it back down? As much as it took to get it there, that energy could have been put into your launch.
Things like lightweight rotational parts don't show up on dyno's that much, but they do in a race.
