I can't tell you what will be better for you, but I can give you my experience.
I had the USPc .40 and sold it. I currently own a P99, also in .40 S&W. My HK was mechanically perfect, but I could n't group with it for beans, and this was after 2,000 rounds. I liked the SA trigger pull well enough, but the DA was very heavy, heavier then others I had tried. Seems there is a bit of a liability issue with light triggers. When H&K changed to a trigger lock built in, they also started using a slightly heavier trigger spring. It should not be a problem to get a GOOD smith to lighten this. I also like the ability to to put the arm 'on safe', decock, or go to cocked and locked. You do not get options 1 or 3 with the P99.
The guy I sold it to shoots it very accurately, but gets occaisonal feeding problems. I think it is due to limp wristing, as he has the same problems with 1911 format pistols.
The P99 I have now has a superb trigger and shoots incredibly tight groups, when or if it fires. Mechanical reliabilty stinks, but to be fair, I only have about 1,000 rounds through it, and it is getting better on the misfeeds, but the slide continues to lock back when there are still rounds in the magazine. If I still have the same problems after 2,000, I will have to make a decision.
Overall, my P99 feels better.
I agree that all polymer frame pistols will feel top heavy. If this is bothersome, my strong suggestion is to go with a Sig 229. I have one and prefer it to either the H&K or Walther. Better trigger, balance, accuracy, never ever a mechanical problem. I do like the ease of maintainance of the polymer and the tennifer on the P99.