I have three Glocks - the 22, the 21 and the 30. If Glocks are too blocky for your hand, send them to Robar and get the Glock grip reduction. After that, a Glock will actually fit the human hand.
I also have a H&K USP45 full size. I have carried all four of these pistols and feel like I am well protected by any one of them. The H&K costs more than a Glock, but a Glock with the grip reduction is in the same ballpark pricewise as the USP45.
If you compare the Glock 21 and the H&K USP45, you are comparing apples and oranges - the actions are totally different. If I had to pick one, I'd probably pick the Glock 21 with the grip reduction over the H&K USP45.
Why?
1: The USP magazines cost nearly TWICE as much as mags for the Glock 21 [[
BIG factor]].
2: The Glock mags are 13 rounds vs. 12 for the full size USP.
3: The Glock's trigger pull is consistent - no DA first shot, SA all others.
4: The USP's DA pull is very heavy, although it can be improved by a gunsmith that knows how to work on the USP line of pistols.
5: Glocks are the easiest auto to learn to shoot well in the least amount of time.
6: Glock is the gold standard for reliability.
7: Glocks are much like revolvers in their operation - very straightforward, no external safety, no decocker, etc.
8: At any major gun show, you will most likely find 1 or 2 Glock trained armorers who can do any number of modifications to your Glock while you wait, including installing tritium night sights.
As for the USP45, you can carry it as a DA auto, or carry it in condition one (cocked and locked) like a 1911, avoiding the heavy DA first shot. It is the only auto that gives you both DA and SA options.
IMO, you can't go wrong with a Glock or an H&K; it it a matter of preference. Either will serve you well.