Just a quick point. A longer barrel and longer sight radius does not make a particular firearm more accurate. In fact, the longer the barrel the less accurate they can be because a shorter barrel is stiffer than a same diameter longer barrel and less stiff equals more harmonic variability, which yields less accuracy. However, in the lengths we are talking about in the real world with average Joe Blow shooter it is a mute point.
The longer sight radius only allows an average shooter to shoot a pistol more accurately; it does not make the gun more accurate and in the hands of a really good shot, that person will be as accurate with the shorter sight radius as he will be with a longer one.
When it comes to the USP being more accurate than the USPC, or the USPC being more accurate than the USP, it all boils down on how the pistol in question fits the shooter shooting it. The pistol that fits the shooter better is going to be more accurate in the hands of that particular shooter. Dollars to doughnuts if the Compact fits me better I will shoot better with it, and if the USP fits you better you will shoot better with the USP. Inherent accuracy between the USP and the USPC is six of one, half dozen of another.