There are 4 factors to a projectile (bullet) wound. They are Penetration, Permanent Cavity, Temporary Cavity, and Fragmentation.
Because of the speed of handgun ammunition, only penetration and permanent cavity are a factor. Pistol rounds move too slowly to achieve consistent fragmentation or to cause damage through the temporary wound cavity.
In order to achieve incapacitation, the central nervous system (CNS) must be destroyed. The CNS is the brain and upper spinal column. In order to destroy the CNS, a projectile must pass through the area (not just reach it). Hits to the heart-even multiple hits-will not quickly incapacitate your attacker. Even if the heart is severely damaged, your attacker will have approximately 15 seconds of full voluntary function before bleeding out.
The bottom line is that you want a defensive round that will penetrate 12-18 inches. The bigger the permanent wound cavity the better. Determining how much better a .45 is than a 9mm is extremely difficult and subjective. As long as the penetration is there, the only thing that matters is placement.
I wrote a paper on this awhile back with more detail about the factors of wounding, etc. You can download it if you're interested:here.