1. Accuracy out of the box?
HK P7M8 most definitely with the Sig 228 slightly ahead of one of my numerous Glocks in 9mm for readily available combat arms. The HK P9S in either 9 or 45 for the most accurate out of the box production type pistols.
2. Concealable?
Your choice there. It depends on body build.
3. Reliabilty?
All have been totally reliable without any FTF or FTE problems whatsoever
4. Cost v. "bang?" (aka "bang for the buck")
Glock wins there.
5. In your experience...which would you buy?
It really depends on the caliber. If it was 9mm I would go with a P7M8. My P9S models are more accurate but are in collector status pretty much.
If money was an issue then I would get one of the Sigs - probably another 228 with a few high caps or a 226 for a bit larger pistol. I love my Glocks, mind you, but the Sig is a great pistol - I am slowly warming to them
I said above IF it was a 9mm. If it was .45 caliber I would go with a Glock G30 all the way. I have shot the Sig P220 but was not happy with MY performance with it; therefore, I don't own one. That could change of course.
I simply can't shoot the HK USP models effectively. But, my HK P9S in .45 is truly the most accurate factory handgun out of the box that has ever been produced(not my claim...but I would agree). It continues to impress me and to others who have been fortunate enough to shoot it!