Can't compare both of them, But I did have a USP .45 for a long time that was a hell of a pistol, but wayyyyy too big for my hands. Yeah, probably doesn't help you at all.

I agree with Rupert that the H&K 45's are bulky for sure....

The recoil between the two are quite similar. The major benefit that leads to the HK45 is the improved ergonomics, they simply can't be beat. In addition, the longer mag release improves overall weapons handling. The HK45 is well worth the premium.

A friend of mine had one and said that the mag release would occasionally activate while in his blackhawk holster and drop the magazine during carry. Is this common?

I used to carry and shoot the USP 45, great gun. It is a bit bulky, they both are.
There is a larger holster selection for the USP.

Recoil is the same. The HK 45 is a Rolls Royce compared to the USP 45. Trigger, interchangable grip pads, o-ring barrel, phosphours sites, and on and on. The two pistols are not comparable except for the caliber and recoil. Get the HK45.

I have a P30 & a HK45 not the c, all are great and cant be compared to the 20 year old USP design. The holster thing, there is no reason for that other than bad design I use Comp-tac stuff iwb and owb, never a problem. The only problem I have with these guns is riding the slide stop.

Good Luck
Kenny

How is the P30? What is it like vs. the P2000 and USP 9mm? I'm mostly a 9mm guy and I've been tossing around the idea of picking up an HK, and I've heard their .45s are like nothing else...but there's the cost of ammo, controlability, commonality of cleaning supplies and ammunition...

In my opinion, the P30 and the P2000 are quite similar, with the main difference being the frame. As you are aware, the P30 has aggressive grip texture and can be rather uncomfortable when carrying IWB. The P2000 has a relatively smooth frame and dimensionally, is a tad smaller than the P30. The P30 is by far, my favorite HK but the P2000 is no slouch, either. For hot summer carry, the P2000SK 9mm is my main carry sidearm.