I've been looking into this topic for a while now, and probally will be doing a clutch soon.
From what I understand, if your car is setup for drag, running slicks, billet steel is the way to go because the heavier flywheel will carry more kenitic energy/mementum and will get you out of the hole better. It is heavier than stock from what I have heard, but never held one to tell you.
Aluminum will weight less, and everyone knows rotational mass isn't good when RPM's are climbing rapidly. I think on street tires, the aluminum would be a better choice.
Then there is the stocker, lighter than billit, heavier than aluminum, and alot cheaper. You can either get your stocker machined or get a remanufactured one for about $60-80.
I'm kind of caught in the middle of the three myself. I would go aluminum, but I am wondering if they are worth the money.