Depends on what you want from your car. Generally, Steel flywheel is better for drag racing and aluminum flywheel more for roadracing. Aluminum flywheels are about 1/3 rd the weight of steel fw. I opted for an Aluminum flywheel and aluminum drive shaft in my Mach-1 because I roadrace it a few times a year. Aluminum fw will rev quicker, but steel fw has more inertia and will be a little quicker off the line. How much, I really don't know. Aluminum fw are a lot more expensive than steel fw.
Wow, I typed this before I read a67tempest455 comments. I'm such a noob.