Brett,
Steel used to be the ONLY clutch plate material. The manufacturers started using aluminum to make bikes lighter without consideration for durability. That said, my alumimun plates have lasted far longer than I ever expected. Steel will last longer, work better when hot and not polute your trans oil with residue. You will notice a slight increase in flywheel effect. The clutch turns 5X slower then the flywheel and since the inertia (flywheel effect) is based on the cube of the rotational speed (physics), so the effect of more weight on the clutch is far less than if it was at the flywheel. I plan on putting in some steel plates in the next few weeks. Hope this helps. Cam.