The pk380 is a wonderful gun, as long as the right ammm is used. My wifes shot S&B hardball OK, but federal, remington, and armscorp was less than stellar in performance. I started realoading practice ammo, using berrys plated HBRN 100 gr, over 3.1gr universal clays, and it seems to be smooth sailing in that pistol. Hollowpoint ammo I have yet to find anything that hasnt fed in it just fine.
The Bersa Thunder I have not had the pleasure to shoot, but I have shot the ppk/ppks, I believe they are very simmilar in size/ pattern, and comarativly, the PK380 is a MUCH softer recoiling pistol.
If I were looking at the same pair, I would look at what you intend to use it for, the PK380 is a bit bulkier than the Bersa, might be a little more difficult to conceal. Holsters for the bersa may be a little easier to find.
Heard lots of good about both, so might be PRM is on to something, you could always just get both.