Both guns were designed as .32s, and are at their best in that caliber.
The Walther is a stronger design. It will stand up to the .380. It's not a lot of fun to shoot in .380, but it will last. The PPK/S is a slightly better choice, in my opinion, as the grip is slightly longer, allowing a better purchase. The PP has the same grip length as the PPK/S, and a longer slide.
The Mauser is more delicate. I would stick to .32 in that one, there are reports that the .380 will wear much faster.
I know you would like to stick to those two for now, but what would the internet be if people followed rules ?
In my opinion, when it comes to a choice between Walther or Mauser (or SIG/Sauer) Beretta makes the best .380s