Regarding the Sig 220, that's totally different animal than the others you are looking at. All the others are polymer framed, higher capacity, striker fired guns. What I like the 220 and the extra heft of the steel frame helps mitigate felt recoil which is bonus, but it only holds 8+1. If you go the Sig 220 route, there's even more choices to be made as Sig has about 20 different "packages" available to choose from. I have a G21SF and had a 220, I preferred the way the 220 shot compared to the G21SF.
Regarding the HK, you should also be looking at the HK45, kind of like a USP but has some nice added features that the USP does not have. I don't know why but the USP has always felt like a toy to me...
The HK45 only holds 10, while the USP holds 12 so it would be middle of the capacity spectrum of what you're looking at but as it's still more than a 220 I don't think it should be an issue to you.
If I were to buy another polymer .45, it would probably be the XDM45 or the HK45. The only reason I would opt for the XDM over the XD is that I all read have a holster that will accommodate the XDM45.
The Sig doesn't really do anything for me that the 1911 doesn't. Although I had a SAO model. If I were on the market for a single stack, DA/SA or DAO .45 the 220 would be it.
What role is this gun to play? The G21 is a little thick in the slide for IWB carry while the 220 is relatively slim and carried well for me. This is all subjective to body & holster type of course.
What are your need as far as weight vs. capacity? How do you feel about safeties? Pending on what variant USP or Sig 220 there are safeties to consider and one thing I really hate about the XD line is that in order to cycle the slide the grip safety must be pressed in.
All the guns you listed are great guns, the HK USP and Sig 220 have great reputations for reliability as well as the Glock, the XDM is newer on the block but Springfield has good rep as well.