Bimmer,
All five makers you've mentioned produce quality firearms that should serve you well- not a stinker in the bunch. For this reason I tend to go along with the folks who are saying that you should handle each of these before making a final decision. Individually, I would add the following:
Sig- I like 'em, good accuracy and nice triggers, but I would recommend going with the milled receiver vice the stamped receiver. The aluminum frames are a cause of concern for some folks. I don't see them as a weakness, but it is something to be aware of if you view this as a potential problem.
Beretta- This is one of the most comfortable and accurate pistols that I have shot. Feverdoc's point is well taken though, if your hands are on the smallish side the Beretta is going to feel like a real handful. If your hands are larger the grips are going to feel great, and the overall design of the pistol just seems to soak up recoil. We had some problems in the Marines with slide breakage on the M9, but I haven't heard of this being a problem in the civilian sector.
Walther- Another well made and very accurate pistol that the makers have gone to great lengths to try and make ergonomic for a variety of users (i.e. interchangeable backstraps). Despite their efforts, however, I can't shoot one comfortably because of the geometry of the trigger and the trigger guard. No matter how I adjust my grip, actuating the trigger always causes the bottom of my trigger finger to scrape along the inside bottom of the trigger guard. I've heard others complain of the same thing. Perhaps this would be a better choice for someone with smaller hands.
Glock- I think this pistol's reputation for being hell for tough is well deserved. It boasts a truly innovative design and can take a licking and keep on ticking. I've enjoyed the ones I've owned and found them to be decently accurate (the trigger pull being the minor weak link here), but I could never really get 100% comfortable with their patented safety system. Even when used with a holster that completely covered the trigger area, I was still a bit hesitant to head out into thick brush with a round in the chamber. If I were a peace officer I'm sure I would feel differently about this.
H&K- IMHO, the USP may offer the best mix of features of all the pistols mentioned. As you're probably aware the pistol can be quickly and easily modified by qualified smith for a variety of different firing modes (DA/SA, DAO, etc.,etc.). I happen to like the standard DA/SA mode as it most often comes right out of the box. The standard mode also allows you to engage the safety while the hammer is down (thereby temporarily disabling the DA mode), or you can engage the safety with the hammer cocked (cocked and locked), or you can use the same safety lever as a decocking mechanism (as with the SIG pistols). Lots of versatility here. I'm also a big fan of their magazine drop levers which are both ambidextrous and conveniently located at the junction of the lower trigger guard and the grip. Actuating the magazine release with your trigger finger takes a little while to get used to, but once you do you'll find that it's safe, it makes for lightning fast reloads, and you don't have to change your hand position on the grip even a little bit to effect these magazine changes. Add to that the toughness and durability that H&K is known for, and you have a real winner.
Good luck in choosing, but rest assured that you'll be picking a winner with any of the above.
------------------
Semper Fi
-Bill