dyezak, great story, thank you for sharing it. I've had some very good experiences with HK USP's and the Compact versions, but as I've said before, HK's idea of a "compact" weapon in the USP series is about the same as everyone else's normal (or service) size. In my opinion, folks with larger hands such as myself won't have too hard of a time on the HK's controls, and don't let the rental gun get you down on HK, generally, those things are completely reliable and built like tanks (though having them serviced can be an annoying experience -which, in all fairness, can be an issue with Sig Sauer as well). If it weren't for Smith and Wesson performing service issues on most Walther P99 problems, Walther would probably be in the same boat. Springfield Armory and Smith and Wesson have some of the absolute best customer service in the industry.
You're right about the Sig, the trigger on the P229 and like variants is amazing and pretty much flawless in terms of how a DA/SA pistol should be. I gather that since you said the Walther's trigger was good in SA mode that it was an AS model, and if that's the case, the trigger gets almost godly after about 400-500 rounds, but it's still not as "crisp" as a Sig in SA mode.
All of the guns you looked at are quality firearms and you'll be good to go with whichever one you ultimately choose, though I'll throw one monkey wrench into your dilemma if you don't mind. You said in the beginning that you were leaning towards the HK, I would suggest that while the USP is indeed a fine handgun, it's not quite as suited for concealed carry for some folks, and a better representation of an HK carry piece would be their P2000 model (
http://world.guns.ru/handguns/hg85-e.htm), which is similar to Walther's P99 (though I prefer the P99 and the HK is generally more expensive). The P2000 is little lighter. Another HK model to keep in mind is their new P30 (
http://world.guns.ru/handguns/hg198-e.htm) which is only slightly heavier than the P99 as I recall. The magazine release on those two guns is similar to the P99's (almost exactly like the 1st gen P99's but shorter than the current gen models).
Also keep in mind that the Sig isn't too heavy compared to some of the real heavy hitters in the semi-auto world (such as the govt model 1911) and with a good holster and belt combo, you should still be able to carry it no problem.
Whatever you choose, out of what you listed, I have no doubts that you'll be served well.