If there comes a time when you think you are really about to have to use it, whatever you picked you will probably be wishing it was more powerful and carried more ammo in the mag.
There is no such thing as too much as long as you can use it well.
Personally my choice changes primarly based on if I am in an urban or a rural environment.
In the city, a double stack 9 or a single stack 9 with a spare mag seem to me to be enough. Why? In the city you can run to help, aid will come sooner or later, there are other targets to occupy the bad guys' attention, and the bad guys aren't likely to loiter.
In the sticks, I want a service size double stack .45 with spare mag at least. Why? No where to hide or run to, no help coming nor anyone or ability to call for it, there are no other potential victims to distract the bad guys (who are likely armed with long guns), distances are likely to become longer, and the bad guys have all the time in the world to finish whatever they intended to do in the first place.