The Sig P-232 is an excellent gun, smooth and easy to shoot. I also have a Walther PPK/S, and prefer the SIG. You might also take a look at the Sig P-239. Just a little bigger than the 232, and it is a 9mm with an 8 rd mag. Shoots a hair better than the 232.
If size is not very critical, I'd go with the 239. Easy to carry with a good holster, on the belt or IWB. The 232 is smaller, rounder, sleeker, and easier to slip in a jacket pocket or to conceal under any kind of dress code.
I don't want to start a caliber war, but there is a lot of evidence that the .380 ACP, especially in JHP, is more than adequate for self-defence. It is a short 9mm (9x17mm, compared to the 9x19mm Parabellum), and will do some damage. Obviously, as you go up the scale to 9mm, .40 S&W, and .45 ACP, you can do even more damage. On the other hand, a .380 in your pocket beats a .45 in your desk drawer. The best gun/caliber is whatever works for you.