I'm sort of new here so I don't know if this ground has been specifically covered, but here goes. I have had an opportunity to spend some time with both a P2000 and a USPc (both in .40 and both with the LEM system), and I would like to post my impressions and see what others' experiences are.
This version of the USP compact has the LEM trigger system, which is basically a single action trigger masquerading as a double action. The effort to pull the trigger back from the rest position is negligible, and from there the trigger pull is the same for every shot. It's fairly crisp with a reasonably short reset. I like the sights, and the grip is comfortable to me. The only downside to the USPc is the proprietary accessory rail, but Picatinny adapters are available for around $20. It's not something I use often enough to worry about for the most part.
The P2000, as you know, is the "improved" successor to the USPc. It's basically the same size and takes the same magazines, but it's got an ambidextrous slide stop, interchangeable grip backstraps, and a universal accessory rail. The P2000 is also recontoured a bit overall - the shape of the slide and frame are noticeably different from the USPc.
Despite all these changes, the only improvement that made a positive impact to me is the Picatinny rail. I can't find a backstrap that makes the gun feel good in my hands, though part of that is the weird shape of the front of the grip which looks like it was designed for a three-fingered space alien. A Hogue Hand-All improved the feel of it for me, but not enough to make it as comfortable as the stock USPc grip. The ambidextrous slide stop I could give or take; I am accustomed to slingshotting the slide so I normally only use the slide stop for locking the weapon open. The front sight doesn't fill up the rear sight notch nearly as well as it does on the USPc; as a result I feel like I can't aim the P2000 as precisely as I can the older gun. My groupings tend to bear this impression out - my average scores are consistently higher when I use the USPc... with one exception. I spent about three months shooting a P2000 on a frequent basis, and at the end of that period I was shooting at 98-99%, score-wise. But after that, when I was shooting less frequently, my scores worsened dramatically (for me anyway), dropping to 90-95%. So given equal amounts of practice, I shoot better with the USPc.
The LEM trigger on the P2000 doesn't feel as nice as it does on the USPc. On the P2000, the trigger feels grittier and overall less crisp and the reset is longer. I don't know why any of that is, considering they're basically identical from a mechanical standpoint. Additionally, the ledge on the slide stop is more generous on the USPc, which makes it easier to use either for closing the slide or locking it open.
Also, I'm not fond of the P2000's looks. I don't know what it is, but the countours (of the slide especially) turn me off. Where the USPc's appearance is all business, the P2000 looks like it's trying to be modern art.
In conclusion, the USPc is the gun for me. On top of all the above, the USPc has never given me a moment's trouble with any kind of ammo, whereas the P2000 starts choking when I'm using dirty-shooting frangible rounds, after about a hundred rounds or so. It works fine with defensive ammo, but there's something in there that makes it not quite as reliable as the USP compact.