SIG is my second favorite pistol, and the P220 is one of the best .45's ever (right behind the 1911 {all praise be to John Moses Browning}). The ST is even stronger than the alloy frame, in fact, Gray Guns has converted several to 10x25mm. I would keep it, after all concealed-carry isn't everything, I don't know if you hunt or hike but an open carry gun has it's uses too, and with good ammo I wouldn't hesitate to pack a P220 out here in Montana.
You could check out SIGforum for more info on your weapon, and if you were to make the mistake (IMO) of getting rid of it, there are plenty of SIG-o-philes there who would snap it up.