I have a P99 A/S in 9mm, and a USPc in 9mm. I also have prev owned a USP fullsize in 9mm. The USP was just too large for my hand, so I sold it. Great gun, though.
The P99 is more ergonomic than any other gun I have ever held. Also, for me - while the HK USP shot very well - I still shoot better with the P99.
Just my 2 cents. Both are great guns.
Only other gun I have shot as well as my P99 is an FNP 9mm.