Which 320? Sub compact, compact, carry, full size, or long slide? Assuming based on the size of what you're carrying now plus the size of the PPS, I'm going to assume you had meant to say the P320 Sub Compact. For starters I don't get my underoos in a wad over the caliber wars but I will say getting small .40's is a bad idea just like getting a small .45 is a bad idea due to the muzzle flip. But eh, whatever it's for you and not me. To be honest if you like the 320 better, make sure it's the revised version meaning a gen2 because you will actually like it. While I have no experience with that Walther I will say nothing because it's not my place to speak about something I know nothing about; but I do own a 320 Carry and it's an awesome pistol and very underrated that has caught the eyes of departments nationwide, possibly the FBI, and from the looks of things the MHS for the services.
Arfcom tradition says both but I say try both first before you buy and then decide.
Good luck.