[Dynamo]

i don't think it matters too much, with the exception of using like materials together. as for durability, there wont be much difference when it comes to parts that will be taking a lot of impact force.
Aluminum parts are light, therefor they have less inertia, but at the same time they are comparably softer.
Steel (mild steel) parts are much heavier, and so have quite a bit more inertia. this wouldn't be a problem if the steel used was a quality steel that has been case hardened(which in our case, it never is.).
case in point: my RA-TECH steel (mild steel.. sad.. really sad) carrier has the whole from face of the carrier deformed for impacting my Prime steel barrel extension (better steel used). it's gotten so bad, it scrapes the inside of the upper after only 2-3 hundred rounds.
same thing happened to my stock WA carrier (pot metal), so i really cant see any reason to pick one material over another other then cost.
stay away from hammers that have steel bearing on the top where it contacts the carrier, as the bearing is made from hardened steel and will eat through both Aluminum and steel carriers, that is of course unless the carrier is made from a quality carbon steel that has been case hardened(unlikely).

recap -
Aluminum- light, soft, it doesn't flex, it bends or deforms with impact.

Steel (mild steel)- heavy, harder than aluminum, still soft. it doesn't flex, it bends or deforms with impact.

quality steel (good carbon content, case hardened)- same weight as mild steel if not a bit lighter, super hard, flexes, absorbs impact well, will out last the rest of the GBBR. too bad no one cares to make internals with this kind of steel. i guess it would be counter productive to make parts that last.