The P99 is a double/single action. PPQ is striker fired.
I have a P99 and it is a good gun. Gritty, spongy double action pull is rather lame but single action is good. I love the controls. The decocker is excellent and the magazine release is cool and effective. Feels great in the hand. Reliable as a Glock. About the same size and weight as a Glock 19. The factory sights are horrible fragile plastic affairs and should be immediately replaced.
Honestly though... I would skip it and head straight to the PPQ. The P99 has a weird double action system called the “anti stress” trigger and frankly it’s a stupid gimmick, they’re the only company in the industry with a trigger like it and it basically introduces a THIRD trigger pull sensation into the mix and you can research it if you want.
The PPQ has a simple always-the-same excellent and light trigger. Many consider the PPQ to have the best trigger of any striker fired polymer 9mm on the market. There are also more holsters and accesories for the PPQ over the P99. Walther doesn’t market their pistols to nearly the same degree as Glock, but they are well thought of and both the P99 and PPQ are popular in Europe as police sidearms.
I’d go with a PPQ M1, which is the original European variant with paddle magazine release. Awesome gun, if you ask me it’s a WAY better gun than the Glock, it’s just not as popular due to Glock’s massive control of the market and huge advertising campaigns.