Actually, for off-road applications (not moto-x), a steel sprocket is recommended. The problem with aluminum sprockets is they are brittle, so when you hit rocks and such, the sprocket will crack, as oppose to bending (steel). Also, you'll notice that a worn steel sprocket's teeth bend, and the aluminum sprocket will actually start losing the teeth (I've seen completely bald alum. sprockets). Finally, a steel sprocket costs less than half of an aluminum sprocket. If you buy a top notch chain, your sprockets will last much longer (DID X-ring), and instead of buying one $50 alum. sprocket, you can buy 2 rear steel sprockets, and be able to do gearing changes.