I've owned both. I still own my 3913 and 3953 and of the two, I vastly prefer the 3913 over the PPS.
Now, the PPS is not a POS, or anything like that. If Glock ever decided to make a Single Stack 9MM, I'd point, giggle and state that Walther has had that option for years. Mine functioned flawlessly for 500 rounds. I liked the different magazine sizes and backstrap options, as well as the consistent trigger pull. The one thing I couldn't get over was the trigger guard magazine release. MAN that sucked. It just went against every muscle memory that I had of the magazine release being on the frame. Seeing as how I could never get used to it, I sold it off. That was really the only thing I didn't like about it.
If you are into the tactical plastic fantastic, than the Walther can work very well. Just beware that spare magazine prices are simply obscene.
The 3913 and 3953 (DAO variant) IMHO are the best 9MM guns that Smith and Wesson have ever made. Thin, easy to shoot, and accurate, these guns are just too nice to have. I prefer my 3953 because of the lack of silly safety levers and it's consistent DAO trigger.