I'm pretty sure a big factor that Tesla went with the cast aluminum arm is that it's faster (and easier) to make than the stamped steel version, which has to be made in a few steps. Cutting a bunch of steel tubes, bending a sheet of steel into a U-shape, then holding three pieces while welding them together is a lot more labor intensive then pouring aluminum into a cast, letting it cool, dropping the piece out, then drilling holes at either end. I'm sure there's a lot more to it than that, but I think that's the idea. As far as strength, they're all probably overbuilt enough to do what they do.