The only advantage to the SS freak inserts is that the tolerances tend to be better and they are stronger so you won't crush them as easy. I have both and I just use whatever fits, I don't get hung up on the actual sizes as printed on the sides.
re; stainless steel backs. i have heard that alum. will eventually expand in bore size after so many years of shooting (i think it was over 150 boxes; 2000*150= a long time) but stainless steel will never expand. i could be right, i could not, its just what i have heard.