You can loose a lot of hair trying to wade through all the information out there from self proclaimed experts. I won't pretend to be one. In my mind, common sense dictates looking at actual events and trying to gauge effectiveness based on real world data. As such, I read Marshall and Sanow's "Street Stoppers" and "Handgun Stopping Power". My decisions for carry ammo were made based on that data. Detailed police accounts and coroners reports provide a modicum of truth, in comparison to a genius on Youtube. The information will always be dated, when you take into account the ever changing technology in bullet development.
Previously, someone mentioned shot placement as the be all, end all. True to an extent. No matter how much you train or practice though, violence is dynamic and there is no guarantee of pinpoint accuracy. Defensive shooting is not sniping, target or IDPA. Adrenaline and fear tend to monkey up the works quite a bit. No different than hand to hand combat. Anyone who says they will do precisely this or that in a gunfight has no experience with true violence. Plans go out the window and you must react dynamically.