I take it you like those type of mag releases ?
I have owned both P99s and USPs and while the P99 fits better, I like the way the USP shoots moreso than the Walther. Also, I know firsthand that Walther Warranty department sucks the big one :-( Not sure how HK is in that department though.
If you are not stuck on one of those two guns, I would sooner go with a Sig, Beretta, Glock, or XD...