Mohecus, definitely didn't want to make it sound like .45 ACP Hydra-shoks aren't gonna hurt! Thank God I don't know from first hand experience, but I imagine it's a tad bit worse than getting a skinned knee...hmm...
The points about the physics of the bullet are important, however. A .45 caliber round, shot into the human body in an appropriate area, can likely cause incapacitation (a nice euphamism for death), but of course the question is: how quickly? Shot placement largely determines how quickly a BG will be incapacitated-- central nervous system (CNS) hits to the brain or spine are the quickest stoppers (just about instantaneous), and shots to center of mass (COM) are aimed at 'rapid exsanguination' or 'hypovolemic shock' (also known as bleeding to death quickly). After shot placement, the next most important element in incapacitation is how much tissue is destroyed and damaged, which is measured in terms of permanent wound channel and temporary cavitation (temporary cavitation with pistol rounds is usually negligible). This is where the hollowpoint v. full metal jacket debate enters (expansion vs. penetration). In general, expansion is preferred, especially since a good defensive hollowpoint round should provide good penetration anyways. Another measure of tissue destruction/damage is energy transfer, which is more in the realm of physics to me than the realm of wound ballistics, but it makes some people happy that this bullet carries 500 ft.-lbs. of energy instead of 450 ft.-lbs. of energy. Lastly, I might consider psychological factors (i.e., muzzle blast, realizing one has just been shot, presumptions on what is expected when one is shot) as a factor of incapacitation, but assuredly not a reliable one.
I guess the bottom line is that .45 Hydra-shoks are gonna make big ugly holes (unless the BG is wearing a ballistic vest), no matter where he/she gets shot, but the importance in selecting a defensive round is how quickly the round will stop the BG or how badly it will damage him. The difference between life and death in many cases is measured in inches or less. Remember: the bullet that Hinckley shot at Reagan was mere millimeters from his heart, Reagan was sure lucky to have survived...thank God Ronny was a tough ol' bastard...
In any event, I've rambled and written a damn dissertation here, most of what I have mentioned has been addressed a dozen different times on this forum as well as others. If penetration is the issue, go with hardball 230-gr. .45. If it's expansion you need, then go with Hydrashok, Gold-Dots, Black Talons, or whatever JHP feeds most reliably in your gun. Doesn't make much difference in a life-or-death situation whether or not a bullet penetrates both sides of the BG's body, what matters is where in that BG's body the bullet is placed.
I think the even better question is: is it better to use a round that has less felt recoil and less muzzle flip (like a 9mm, or .357SIG) that can put more rounds on target faster, or a heavier round (.45ACP, 10mm) that puts bigger holes in an assailant at the cost of bigger recoil?