All surplus FAL mags were made on FN tooling, so they all work well assuming they haven't been beat to hell. I like steel better because they reliably drop-free when empty and I hit the button, but I've heard some people can get the aluminum to do the same. Aluminum is MUCH lighter than the steel, but with 20rds of .308 in it the mag weight is not that big a deal, and the steel is a little heavier duty.
Both are good - I agree, get some of each!