Just based on the fact that the HK45 was designed as an improvement/replacement for the USP should make you choice pretty easy. Their experts looked at all the shortcomings of the USP and addressed them in the HK45.
I've handled, inspected and shot both side by side and for me the HK45 wins, hands down just for the improved ergos...plus I love the look of the grips
I remember the story quite differently. Heckler and Koch had teamed up with Larry Vickers and Ken Hackathorn to create a new pistol; at the time the rumor was that HK was going to build a 1911. What emerged was the HK45, which was synchronized with the Gov't solicitation for the Joint Combat Pistol program. I'm quite sure that with Vickers being plugged into high levels of the Military community, the format of the HK45 was not coincidental.
HK has stated that they have no intention of discontinuing the USP series. The HK45 is considered by them to be a product improved version, but at the end of the day the JCP program was the primary target.
The "new and improved" buffer recoil reduction system does not work as well as the dual-captive recoil assembly found on the full size USPs. The fact that the HK45 has a lower profile slide that sits lower in the hand is offset by the less efficient recoil reduction system. The HK45 feels less blocky in the hand, but in my experience in letting new shooters try the USP45f, they admit that it is a "big gun" but they usually don't complain about the grip. I think this is a learned behavior from those of us with more handgun experience. One big negative is the trough at the bottom of the trigger guard. It rubs my trigger finger raw. Even Larry Vickers has come out against this "feature" and recommends removing it via Bowie Tactical Concepts. He said HK threw that in at the last minute and that he disapproves of that move. My raw trigger finger agrees.
I've had the pleasure of shooting & reviewing one of the first prototypes of the HK45. It was an exciting offering at the time, that's for sure. Shooting it side-by-side with the USP45f makes you appreciate the USP more and more, not to take anything away from the HK45.
Both are great guns. The USP45f has been at my bedside with a TLR-1 for the past 10 years. I have no intentions of replacing it with anything else that has been offered in the marketplace since.
___________________________________________
Between the HK45CT and the USP45CT, I think the newer HK45CT is the winner. It is more sleek and since both of those pistols use the same technology internally (unlike the HK45 vs. USPf), the lower sitting slide with less mass makes it a bit nicer to shoot.