For about the same size as a Derringer, I'd go for the Kel-Tec P3AT. The price is right, the bugs are long-since worked out, and it's controllable. And about half the size of a snubby. I don't own one, but I plan on getting one! But if you opt for the Derringer, the advice about using Glaser Safety Slugs is worth heeding. A lot of the cops I used to work with carried little Beretta 25's with Glasers, and at bellygun range, they will do a number on a bad guy!
Papajohn