Will a stainless last longer? Probably. Do I really worry about that? No. I've had a number of SIGs, all alloy frames. Some new, some from the 90s, and some from the 80s. They've all been fine. As has been said by the time you "wear it out" from shooting you'll have spent more than ten times its value in ammo. I do recommend a good grease on the rails.
Guns don't kill people. Apes with guns kill people! - Robin Williams