i got a set of both and honestly don't see a difference. i have the Smart Parts All American front and Freak back with the viewable cut out window, so i tend to use the SS inserts...just so i won't have a colored insert showing through on a black and red gun. i use the aluminum inserts on my Deadlywind FIBUR barrel cause i can't see the insert.
also, i have yet to dent or bent anything on the aluminum inserts or have seen any scratches on my SS ones...but i do take care of my gear pretty well.
i would go with the aluminum ones, since like Nerve suggested, they basically do the same thing just ones cheaper. i just got the SS ones for cosmetic purpose.