Actually the UR Aluminum Flywheel uses a billet steel center section inside the aluminum flywheel.
This gives the best of both worlds.
I would stay away from an straight aluminum flywheel. It could not dissipate heat, nor have the longevity required for day to day driving. (Stop and go traffic is murder on the clutch/fly/plate)
Plus if you get too light you will have idle, acceleration, and drivability problems.
