Hi twoblink;
I'm left-handed.
I own the Glock 23 and the H&K USP .45 Compact. Before anyone gets upset at my opinions, let me say that I own, and have owned, multiple examples of the guns to be mentioned (H&Ks, Glocks, SigSauers).
The USPs I have are in Variants 2 & 10, making "cocked & locked" carry an option for us southpaws, without the necessity of an ambidextrous safety. It works as designed. The USP's mag release is ambidextrous, not that it matters. (The typical position for the mag release button on most semiauto pistols, on the left side of the frame, has never been a problem for me, and I find it easier to depress this button with my left index finger than most right-handed people do using their right thumbs.)
I'd definitely consider the Compact USP .40 S&W over the full size version, for CCW.
Both guns are more hardy & durable than anyone could need. Both are reliable. You can strip the Glock down to the last pin & spring easily, yourself, while the USPs are very difficult to take down beyond the "field-stripping" level.
The Glock has an "always-the-same" trigger pull, which is easily handled. The USP has the traditional DA-first shot, subsequent shots SA setup in most variants, with the option to carry cocked & locked, as noted above. The DA pull is not especially smooth or short, and is inferior to that found on most SigSauers, IMNSHO. It can also be had in DA-only form, but who'd want such a thing?
Frankly, unless your specific intent is to carry cocked & locked, I'd go with the Glock. Fighting that first-shot DA trigger pull on the USP is NOT conducive to an accurate first shot, and then you have the transition to SA to contend with. I know that may not seem important, but if you run speed & accuracy drills, you'll find it makes a big difference.
Another point has to do with the height of the bore axis above the hand. The USPs, like the SigSauers, have an exagerratedly-high bore axis relative to the position of the shooting hand, making for enhanced muzzle flip, due to the larger moment arm. The Glock sits lower, and the improvement in muzzle flip, and therefore in speed of follow-up shots, is certainly noticeable.
You can purchase "pre-ban" (13-round) mags for the Glock 23, if desired, at a modest cost, and standard 10-round mags sell for much less than the standard USP mags (around $40-50).
The barrels of BOTH guns have polgonal rifling, though the twist rates probably aren't identical. Good examples of either gun are highly accurate, without a discernable difference in my hands.
Next there's the finish to consider. The H&K USP "Hostile Environment" finish on my USP slides isn't all that tough, and shows considerable holster wear at "high points" with only a little carrying, and in spite of assiduous care. The Tenifer finish on the Glocks, on the other hand, is almost impervious to wear, and even the phosphate finish covering the Tenifer wears well. My 15 year-old Glocks still have almost all their phosphate finish, and the slight wear evident at high points adds character.
On the other hand, I know people with smaller hands, for whom the USP Compacts are a perfect fit, while the Glocks are just too large. Obviously, if one pistol really fits your hand well while the other doesn't, your choice is made. If there's no big difference, I'd recommend the Glock.
Best of luck in your choosing.
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"Potius sero quam nunquam."