Have both. Here's my long comparison.
Caveat: Have been shooting Glocks for 12 years, USPs for 1.
RE: Trigger
My USP9 is V9 (safety only.) When I got the USP it was V1 but I never intended to use the DA mode because the DA pull really sucks compared to a Sig for example. But even using SA only I also had a couple problems with the V1 trigger mechanism hanging up due to partial decocking while riding the safety. After getting the V9 control plate, I installed it in about 2 minutes. Some people think the ability to switch control modes is a big deal, but once you find the mode that works best for you, you're done with that "feature." The way HK pushes this feature they make it sound like everybody changes firing modes as often as they change their underwear.
I also have a USP45F with the $100 "Match" trigger and while smoother in DA mode I don't think it really makes that much difference. I can recommend a $5 10-lb (reduced power) Wolff hammer spring though. Makes a big difference.
Going from cocked-n-locked, each shot from the USP9 is more precise (for me) due to the light SA pull. However the trigger reset is longer and it's harder for me to shoot the USP fast.
Re: Accuracy
In addition to the SA trigger, it's my impression that the USP just groups a lot tighter. My bullseye targets are always at 25 yds and it's much easier to keep the rounds in the black with the USP (standing, unsupported.)
RE: Magazines
The USP9 magazines drop free every time. If your USP9 mags don't drop, it's probably because somebody bent the hammer strut trying to force open the gun when the built-in lock was enabled. A bent strut rubs on the magazines, a straight one does not. My USP9 had this problem (no it was not me who bent it) and I got a replacement hammer strut for $5.
I've always found fully-loaded Glock mags to be difficult to insert with the slide forward. They always have to be slammed home. USPs don't have this problem.
I have no problem using the HK mag release, it requires less grip shifting than the Glock release. The release can occasionally pinch the finger below the trigger guard.
RE: Feeding
I haven't had any feeding problems with either the Glock or the USP9. I've shot everything up to +P+ and NATO rounds, and they both work great.
RE: Maintenance
Here's the big difference in my book. I can take down a Glock with just a 3/32" punch, get any part I need from a variety of sources on the 'net for cheap, install it myself and it will work just fine.
RE: Accessories
Glock is much better-supported here. But neither gun needs much in the way of accessorizing. You can get just about any holster made for both guns, with or without the relevant tactical lights (M3 for Glock, UTL for HK)
RE: Durability
If there was ever a 9 that could theoretically outlast the Glock I think it might be the HK. Not that anyone could actually do it... but everything about the HK seems over-engineered for the 9mm.
I did manage to get part of my HK to rust. The little pin securing the hammer spring/strut on the side where it contacts my skin. I sprayed some CLP on it and wiped off the rust.
Some have claimed they got their Glocks to rust, but I have a hard time with that one unless they sanded the slide down deep enough to get below the Tenifer.
I haven't heard any credible reports of anybody kB'ing either the G17 or the USP9.
RE: Ergonomics
I find the USP9 to fit my hand better. YMMV
I've been trying to shoot isoscoles and am finding that my right thumb wants to rest right on the Glock slide stop, which is not good. However the same thumb rides on the USP safety, so I don't have the failure-to-lock problem.
The USP slide stop is much easier to use as a slide release because it's massive.
RE: Capacity
I assume you'll be using pre-ban mags because these guns are really a bit big for 10-round guns.
Keep in mind that the G17 holds at least two more rounds, and +2 extenders are easily available.
RE: Overall
Overall I find much about the USP9 to be more refined than the Glock, specifically the trigger and the magazines. This refinement helps me on the 25yd bullseye range but when the goal is A-zone accuracy combined with blazing speed, the Glock works better for me, primarily due to the low bore axis and shorter trigger reset.