I don’t have any experience with the P99. I do owe and consider my PPS in 40 my primary carry gun. It’s extremely easy gun for CC. During the summer I carry mine in a Full Moon inside the pocket holster. One down side is your capacity option are 5,6 or 7 rounds. For me, the small capacity was fine. I mean how many rounds of 40 do ya need. It’s very enjoyable to shoot was well. I’ve got around 300 pumped through it with no jams or misfires. Good luck in choosing your Walther.
I own the P99 AS in 40 S&W. It's been a great gun and I'm extreamly happy with it. The only down side to it is the size which makes it hard to conceal in the summer time. The P99 is available with 3 trigger pulls. AS(Anti-stress) DA/SA, same style as a beretta 92/96, QA(quick action)simialr to the SA pull on a beretta 92/96, and DAO(Dual action only) similar to a Glock trigger pull. The DOA is sold as the P99O. The P99O doesn't have a decocker button.
There is a downside to the QA. Once you load the gun and chamber a round, if you press the decock button, the gun will not fire until you pull the slide back again. If you have a round chambered, you only need to pull the slide back about a 1/4" to rengage the striker. The QA has a very light and a very short trigger pull (.371 of an inch travel distance if I remember right). IMO the QA model is more of a compatition gun or only one you would use going into a situation where you know you are going to firing it at something or someone.
Last month I got a 9mm PPS. I have run about 100 rounds through it so far and like it a lot, 0 failures. Easy to conceal and to shoot. I do not feel the recoil that much. I think the only knock from my standpoint is the magazine capacity, it holds 7. I have shot a DAO version of the P99 as well and liked it. It was bigger than the PPS but still seemed comfortable to shoot. The P99 Magazine I think held 15 rounds. I went with the PPS because it would be easier to conceal but I do not think you can go wrong with either.