I went through exactly the same question...
About a year and a half ago I went through exactly the same decisions. I fired the HK USP40f, HK USP45f, HK USP45c and the Sig 220 side by side alternating magazines. I have reposted certain parts of that post several times, but here is a link to the lengthy post of my shooting impressions:
H&K vs Sig - Shooting Impressions
As indicated, I purchased the HK. After buying the gun, I fired the first 200 rounds through it and wrote up a report on how that went:
HK USP45f - first 200 rounds
Finally, now I have about 2000 rounds through that HK USP. Over time I have installed a match trigger, changed out the external safety for a decocker only, purchased two 12 round "pre-ban" magazines, and added a utility light (cute little thing!) to the utility rails rails built into the gun.
The USP has served me very well. Other than a broken firing pin (flawed and it broke within the first 300 rounds dry-firing), I have had NO failures: NO failures to fire, NO failures to feed, NO failures to extract.
Would I buy this again? Definately. Do I thik the Sig is an excellent gun? Absolutely. In the end, make the decision on how you like each of them, how they fit your hand, and which you feel more comfortable with. For me that meant the more durable design of the HK and its finish along with a 12 round capacity(grey market mags), or 10 round capacity. If the Sig 220 had a ten round capacity, I MIGHT have gone that way based on how nice it shot. But the dirt/dust that I subject my pistol through during remote camping and jeeping left me feeling better about the H&K.
You will find proponents of both excellent guns on this forum; really you can do no wrong here.
JT
as usual, YMMV, IMHO, FWIW, and all that...