I have two H&K pistols- a P2000 9mm and an HK45.
I can't say enough about them, they are great guns.
some people prefer pistols with metal frames but I like the "high speed" qualities of a polymer frame
its a personal preference.
HK's, like Glocks and Sigs, are made to go BANG when you pull the trigger. everytime.
its why they're primarily designed as service firearms.
people rag on them because they're expensive but to me they are literally the BMW's of the firearm world. some people just like Chevy's and some people just like Honda's so I will not bother wasting my time explaining why I like German cars and German guns.
Mk. 23's are great guns too but they are really expensive (around $1500-1700) and VERY LARGE.
if I were you depending on your budget I'd just get a used USP. those are nice guns.
and if you have a little money sitting around get the new HK45. co-designed by Larry Vickers and H&K and based on the 1911 it is said to be one of the the best .45's ever made.
I love mine and would recommend it to anyone.