USP45, without a doubt. I have owned both.
Dimensionally, the Glock is only slightly larger in the grip. I mean, we are talking REAL minute differences. I have compared the numbers from both official websites for grip width and length.
I think the HK is slimmer because they understood that a metal magazine would afford a smaller grip, because the mag is slimmer. The G21 mags are still polymer, and thus thicker although they still hold the same amount of rounds (USP45 hicaps are LEO only in the US, no prebans).
I can overlap my thumb and pointer finger of my shooting hand while gripping the USP45. I could never do that with my Glock 21.
I think the USP45 grip is perfect for me, and my hands are pretty average as far as I know.