two very different models. One is designed for carry and the other is some sort of combat-military-target hybrid gun made for Robocop, for a pistol, I consider the Mark 23 being way too big.
I would recommend that you invest watching at least a few videos online if you are going to order online, just for size comparison. You tube is your friend.
The other thing is, the USP design is a few years old and quite outdated in comparison to the P line. USP has been maintained because they try to resemble things found in most common north american designs, this seems to appeal to some die hard old school shooters in the US market.
The P line is their newest and most ergonomic design. Apple and Oranges really, like 1911 and a Glock pretty much. Two different beasts.
In terms of quality all HK handguns are outstading. If you want a state of the art polymer gun you won't go wrong with either one.
If you want something to compete in IDPA-IPSC your best choice is the P30.