Well brother, I will say you have selected 2 really good pistols.
I have owned the 220ST and have shot several xd45's. I prefer the XD for the following reasons; less weight, better capacity, I shoot it better, plenty accurate, inexpensive magazines, and popularity has grown so more holsters and what not are around.
Dont get me wrong the sig is a very nice shooting pistol. However, the problems I have with the sig are several; the grip is too long from front to back for my hand, the double action sucked, price for a CPO 220ST was around 700.00, mags were 50.00 per and the held 8 rounds, the bore axis was waaaay to high, and the sheet metal controls really did it for me. It was accurate, reliable and fun to shoot. If I were on a HRT team or needed to club a baby seal it would have been perfect because it weighed (I think) 3 pounds. It had a rail if were inclined to hang something off of it like a mall ninja. I tried to ccw it for a few weeks until my hip could no longer take the weight of the "crew served 220" so I switched to lighter pistol.
Depending on what your budget I would personally choose the XD45. By the way there is the new XDM45 coming out as well.