I opened this thread because the thread topic title was vague (no caliber listed).
While I've never been interested in buying a current HK pistol, if I were going to do so, it would be a USP9 compact. I'd much prefer the variant that provided a traditional double action mode (DA/SA).
Granted, I did rather like the P9S .45 I carried off-duty for a while as a young cop, but selection and availability of leather and spare parts were a bit scarce, so I didn't keep it.
Anyway, if I were told I was going to be issued a USP 45, and could choose the model, I'd opt for the compact version, because it feels and balances better than the boxy full-size version, in my opinion.
The various USP's I've observed used, and have used, as a firearms instructor over the years have been decently accurate and reliable for service weapons. The well-worn stainless USP9 in our training inventory doesn't have what someone might consider to be a stellar trigger, but it runs fine and provides acceptable accuracy under demanding drill conditions. I'd carry it if need be.
There are other plastic-framed .45's I'd prefer to spend my own money on, given my druthers, but someone could do worse. Suit yourself and enjoy.