I had a PPK/S in stainless back in the early nineties, imported by Interarms. It was a great piece, and I shot it well. But, it was not that comfortable in my hands; the grip was quite thin. The gun was pretty snappy, but not as bad as my Grendel P10 I also had back then (a predecessor to the Kel-Tec P3-AT.) As I recall, its magazine capacity was six (correct me if I'm wrong on that, but I understand its seven now.) I sold it during some hard times after owning it only a year or so.
I bought a Thunder 380 in 2008, in a much-belated attempt to replace the Walther on a budget. The Thunder is lighter, has an excellent trigger, and holds seven rounds. It's far more comfortable to shoot than I recall the Walther to have been. Its only shortcoming as a replacement for the PPK/S is that it's bigger and thicker, which makes it not as easy to pocket-carry.
The differences in finish are obvious, and so is the difference in price.
For me, I consider the Walther to be worth about $100 more than the Bersa. That is to say, I'd trade my Thunder toward a PPK/S (stainless only) if I only had to pony up one additional Benjamin. That's just me, though.