Metallurgically, the materials are slightly different. In this application there is no practical difference.
FWIW, my KI (1998) stainless USPc .45 slide looks today like it did when new -- no cosmetic, wear or peening issues anywhere. Same for my new-to-me KH (1997) full size stainless USP .45.
I wouldn't hesitate for a second to drop in a .357 SIG barrel into a USP .40, either my standard finish full size .40 or a stainless version.