Steel is stiffer, so if you're planning anything on the heavier side, we recommend steel just because it doesn't deflect. Both can carry the same weight though, just usually want to make sure it is more spread out across multiple crossbars with the aluminum.
The aluminum come with 1/4" aluminum side rails, and the aluminum crossbars, but do include two stainless steel crossbars. Steel is .185" side rails.
The aluminum weighs about 30 pounds less, not entirely significant in the overall scheme of the vehicle. ~50 pounds vs ~80 pounds.
Shouldn't notice any noise difference between the two when talking about complete racks.
We do have steels in stock right now, but aluminums are out about three weeks.
What other questions do you have?