I cannot comment on the newer HK45 c other than it is a different design, some would say an improved design and the ergonomics are superior. I have had experience with the P30 and in most ways the HK45 is the 45 version of the P30. The P30 is outstanding. Options and configurations should be the same or similar between the models. By all accounts I have read it is an excellent firearm.
I do have experience with the USPc 45 and thought it was very good. Good enough I intend to get another one. I don't think you can go wrong with either if HK is your cup of tea. The USPc is a bit blockier especially in the slide but without a doubt a reliable work horse and at a lower price point. Both are going to be solid performers but the HK45 c is going to be more refined, have superior ergonomics (lot of effort went into the new modular grip design) and of course more expensive. Hope that helps.