My own take is that you are not over-supplied with .45s, so why sell either? The P220 is an aluminum-frame gun, lightweight. Your S&W is all stainless, heavier. It’s not an apples-apples comparison. There are times and places I might wear the P220 if I liked it (I prefer a Commander). The 4566 would be a dedicated range gun, where I would not be worried about wear and tear on the aluminum-frame of the SIG. That's my .02, FWLIW.