I had an aluminum fly wheel and went back to the stock one. IMO the aluminum one is good for autocross and road racing but, I like the added weight and centrifugal force of the stock steel one. I could notice a difference at the track. If you do decide to go with an aluminum one get a solid one piece one, not the ones with the insert where the clutch disc contacts. I say spend the money on an aluminum drive shaft.
I have heard you see a small performance gain with the aluminum but they are not as strong as the steel. I went with the steel for the added strength; id rather have the peace of mind my flywheel isn't going to shatter than the minimal gains you will see from an aluminum one...
IF I recall correctly (see what I did there? no acronym)... Ally fly's aren't really worth it until you're pushing 800+ rwhp... even on street/strip cars. Maybe roadcourse trailer queens, but pretty sure the consensus was they are too light for mixed duty and midrange rwhp.
IMO go billet, lighter than the hunk of iron or whatever the stocker is... but heavy enough to not be a pain in the ass. I think for the 1/4 the ally flys were losing time over stock and billet ones up to a certain rwhp?
So basically if you autocross or track your car, the AL flywheel is nice, but for drag racing it isn't so much. For daily driving it's personal preference - some like it some don't.
In general, the lighter the flywheel the faster the motor spins up AND down.