The USP will hold a little more ammo and have the lever style de-cocker. The P2000 will hold a little less and have the push button de-cocker (Assuming we are talking about DA/SA and not LEM guns) on the rear of the slide. as well as an ambi slide stop/release and what I think are better ergonomics, well you probably would agree if you like the SK. If you SK is a .40 you can use the P2000 mags in the SK, not true of the USP magazines.
The P2000 is a little in between the USP and USPC in size, I like the longer grip and the shorter slide, not that he slide length really matters.
The USP is a fine pistol, but not as refined as the P2000 or later guns such as the P30 or Hk45, but some would say that the USP didn't really need that stuff. Personally, I never really cared for the USP which is funny as I love the P-Series Pistols and HK45/HK45C.