IF I am going to carry a derringer I would want the lightest and smallest one possible. The short barrel and lack of grip does the 9mm no favors either. So the mini mag with the extra rounds would be the way to go. It would likely confuse and injure an attacker enough for you to make a getaway.
But why not just carry a snub .357?
It would be a heck of a lot more fun. Although not as cheap as the mini.