No matter which pistol one compares the two cartridges in, the .40 S&W (nicknamed the "Short & Weak" by real professionals) will NEVER match the veteran .45ACP in ANY criteria of self defense.
Rather than deal with amateur opinions, I have found that the professionals, both commercial and independent, have given a slight edge to the H-K, from tests run by evaluators from such sources as Guns & Ammo, American Handgunner, and Gun Tests magazines.
In such tests, the H-K USP models had fewer malfunctions and NO (0) failures when tested against, not only the Sig-Sauer, but Beretta, Springfield, and S&W models, as well.
The Glock has a slight edge in the economy department , especially with so many used units available on the market. It is the simplest to operate and maintain, As well.
I've run nearly 5,000 rounds through my H-K USP .45ACP, including Hydra-Shok, Cor-Bon, and several equivilent custom handloads, with 0 stoppages EVER, and it's tolerances haven't measureably changed since I first aquired it.
The only negative factors I've dealt with carrying the USP is it's larger size, but I'm a larger guy and LIKE the more impressive size, and the limited availability of carry systems, but I carry concealed in my own unorthodox manner, anyway.
It's mainly personal choice, anyway, so go for it!