usp: the more expensive of the two choices, by no means cheap (usually $600+ in most places)...i love my usp, i find it very reliable and accurate, and it has a myriad of variants depending on handedness, dao, ect...decocker/safety allows for cocked and locked...
one knock on the usps, many many people find them rather large to the point of uncomfortability...even the compacts many consider unweildly...
p99: is a bit cheaper than the usp, i've seen it on sale recently at a large chain for $499...the models i've handled were dao (not sure of variants), so it would have a consistant trigger pull...a few bells and whistles, such as adjustable hand-grips and a choice of 3 finishes (blue, stainless, and military green)...has a very good reputation for accuracy...
i've heard some disturbing rumors about jamming which prevented their sale in some areas, but those may be just some urban legends...
if i had my choice, i would choose the usp