I would recommend the aluminum jaws over the brass. The brass will
chip off easier than the aluminum. Meaning if you hit it with a hammer
pieces of the brass can come off. The aluminum can bend but can be
bent back into shape also.
If you are holding a specific object the brass will be a bit softer
and not mark your work as much as the aluminum. If you can get some
rubber or soft plastic and hot glue it to some metal or aluminum jaws
that could help you if you have pieces that you don't want to mark up.