I have several German Walthers, a S&W PPK, and I have a Mini Thunder .40, and had a Thunder 380.
The Thunder 380 was a new gun that had a lot of problems with stovepipes, jams, and FTF. Eventually, the slide fit became so bad "burrs" developed along the inner edge of the slide. I finally had a new slide purchased, had it fit, and that slide cracked.
Afterwards, I sold the Thunder 380 and bought the S&W.
I think people had irrational expectations of the S&W Walthers, which lead to a lot of internet groaning and griping. I've had excellent luck with the gun, and I've been much happier with the Smith than I ever was with the Bersa.
A friend of mine has an older Thunder 380 that has been exceptionally reliable and very well built. It's been a great gun, and if it came up for sale cheap enough, I would not hesitate to buy it. I kept the Mini-Thunder .40 that I had, and I still like it.
I would, however, recommend you go with the S&W because I think it's a higher quality gun that will retain it's value better over time, should you need to sell it or become bored with it.