I have only recently begun carrying a revolver some of the time, and this article at American Rifleman echoes a lot of my thinking on the subject:

If my travels had me passing through more dangerous areas, a powerful semi-auto would be my hands-down choice. Were I a shopkeeper in an area where robberies by multiple assailants were not uncommon, a slab-sided autoloader with spare magazines would be my constant companion. However, for a day in shorts where a belt holster would be uncomfortable, my choice would be an ultra-light wheelgun in a windbreaker pocket.

Perhaps most importantly, the article also mentions that "For most civilians involved in a deadlyforce encounter, the event is over in three shots or less." If somebody tries to mug me walking along Franklin Street, my first hope is that my assailant will flee as soon as I pull the gun. If I do have to shoot, I'm certainly not sticking around for any longer than necessary: fire and retreat.