Here are my thoughts on the subject:
There is no substitute for absolute gun/ammo reliability; so carry a weapon loaded with quality JHP self-defense ammo that always fires when you pull the trigger.
Most civilian self-defense shootings are at close range with 3-5 shots expended. Capacity is a great thing especially considering multiple assailants, but if after 8-10 rounds trying to stop the threat one or more of the determined perps will probably have managed to successfully target the lone defender. Reloading at this point seems to be a pipe dream for the typical untutored civilian like me.
When I visit the shooting range the first thing I do is shoot my carry ammo rapid fire in 2-shot, 3-shot, and 5-shot burst from the cold pistol to satisfy the gun/ammo reliability. This doesn't guarantee things will always go as designed (there are never any guarantees) but it does provide a real world test protocol and provides a great amount of reassurance that if I do my part the chosen apparatus will do it's part.