The PPK/s came about because the PPK did not have enough importation points to get into the US. So Walther Germany placed a PPK slide on a PP frame, making a hybrid of the two models and the PPK/s was born. The PPK/s was made in Germany until Interarms began to produce them in the US. So you can find US and Germany made PPK/s's. Interarms also began producing the PPK as well, since they did not have to deal with importation laws.
The only real difference between the two is the grip panels, the length of the grip itself, and the magazine capacity. The PPK is a tiny bit smaller and lighter than the PPK/s. Which one is better all depends on preference.
Smith & Wesson is making the PPK, but it is harder to find than the PPK/s. The PPK/s is much more popular in the US than the PPK. The PPK is chambered in .32ACP or .380ACP. The .380ACP model is easier to obtain.
I would like a PPK in .32ACP and according to S&W, they are shipping. But according to the shop I buy most of my pistols through, they can not find one. They can get it in .380ACP though.
Frankly I would rather obtain a PPK .32ACP in blued finish, that was produced in Germany. Just my preference.