Both of these guns are great! It is difficult to beat the USP, but it does carry a more significant price tag. On the other hand, the Glock is VERY proven as well. My only problem with the HK is the safety/decocker. I like the fact that if my weapon is loaded and I pull the trigger, it shoots. Too many features can be a bad thing in a tight spot.
The HK will tend to be more accurate than the Glock, but you can take care of that for less than what the difference will be between the two guns.
My Glock 20 is one of the best-shooting guns I have owned. The first thing I did was install a 3.5 Ghost disconnect. The reset is about 1/8" - makes for great follow-ups.
If you install a mag well on the Glock grip, you will find that it makes a HUGE difference. It helps your hand suck right up on the grip. I had one on my Glock 31 and I will now put one on every Glock I ever own!
Overall, you are looking at a difference in price and preference. You may be talking about a 1-2" difference in groups at 25 yards; if this accuracy difference is a problem, you should be shooting at your target with a long gun anyway. So, it comes down to what you are most comfortable with. Handle both guns a bit, and if at all possible, shoot them at a range. You will find out which one is for you when you actually shoot both of them.