Several excellent points have been made about the USP and the P99. I have a full size 9mm USP and I just sold my 9mm P99, so let me add my two cents.
1. HK has a lifetime warrany; Interarms was giving one year on the P99. However, Smith & Wesson is taking over distribution and service for the P99, so this may change.
2. After about 5,000 rounds, the part of the barrel that the recoil spring lodges against broke. Earlier I had two firing pins/strikers break. The broken firing pins were attributed to dry firing, although Walther says that the P99 was designed to be dry fired with snap caps. On the other hand, Walther has changed the barrel (I don't know what was done and they are not interchangeable), and my P99 had the earlier barrel design; Walther has also strengthened the firing pin/striker. If you are interested in the 9mm, find the highest serial number you can. Mine was in the 9000s; the highest I have seen is in the 18000s. The higher the number, the better the chance that you will have the new barrel design and striker. I don't know whether the barrels or the strikers were changed on the .40 S&W. Walther has also changed the trigger on later P99s.
3. For a good price, try Bachman Pawn and Gun
http://www.bachmangun.com.
4. I shoot better groups with the USP. My P99 always seemed to shoot low, even with the smallest front sight. My USP is much closer to point of aim, especially at shorter distances.
5. High capacity magazines are available for the 9mm P99 at about $100 each.
6. If size is important, don't overlook the USP Compact. It is a lot shorter than the USP, and a little shorter than the P99. It also has a smaller grip and a shorter trigger reach.
[This message has been edited by Cawdor (edited August 21, 1999).]