Interesting question, but I think the answer is "unknowable." Whether you need to use ammunition with a high degree of penetration or not depends on the circumstances of a violent confrontation, and generally one is not going to know what those circumstances are until it one is in the middle of it.
A couple high profile police-involved shootings come to mind. Trying to disregard the political sides of these events, or whether the guys being shot at needed to be shot, they may illustrate problem over expansion vs penetration...
- In New York, there was the incident where four policeman, using departmental Win 115 gr FMJ (yes, ball ammo), fired 41 times in a very short amount of time at Amadou Diallo. Massad Ayoob wrote about this
http://www.findarticles.com/p/articles/mi_m0BTT/is_149_24/ai_65910628 and noted that of the 41 shots, only 19 were hits (misses, are, of course, a problem of neither expansion nor penetration). Of the hits, only about two hit where they had immediate affect (the spine). In a later article (not the one I linked to) Mr. Ayoob noted that a number of the hits just went on through the body, and of course the rounds that missed also had little effect, so in keeping with their training to shoot until the threat was neutralized, the cops kept shooting, which contributed to the high round count. Anyone standing behind Mr. Diallo would've had a bad day, but I believe he was standing in doorway or entryway of somekind, with no bystanders. So here's a case where non-expanding ammunition didn't provide incapacitating wounds, and penetrated too much, which may have contributed to "not stopping the threat immediately" and posing a danger to anyone in the background.
- Second incident that comes to mind is a more recent shooting out in Compton (Los Angeles), where 10 sheriff's deputies fired about 120 rounds at a guy in a vehicle. Some shots hit houses in the area, one deputy was hit once in the b-proof vest, and the suspect was hit only four times, and apparently not severely at that. Thus it would appear that most of the shots hit the vehicle, but only four made it to the suspect. (Possible that some others went thru the vehicle but didn't hit the occupant I guess). Here's a case where I think there probably wasn't enough penetration, and again in keeping with training, the deputies kept shooting until the vehicle finally stopped (this last point was not in any of the news articles I read, it is my own supposition).
So how do you know in advance which you are going to need? You don't, is the answer. Even if you have one magazine of super-expando hollowpoints and one of maximo-penetrator, it seems likely that you will not have time to figure out which you need and get it in your gun (Murphy's law - whichever you need will not be in the gun).
I think Mr. Camp's question about penetration may have been more oriented towards penetration of the body, not barriers, but I think my thesis still holds - it is not knowable which is better until it's too late. The best you can do is decide which situation is more likely and load up for that.
I, as a non-policeman, think it is less likely that I will be involved with shooting thru barriers, and given I am in Texas, heavy winter clothing is not common. So, for self-defense I will go with ammo that tends to exand more and sacrifice penetration if I have to. If I take up feral hog hunting, which has tickled my fancy on occasion, I will definitely be looking for something with more penetrating power!
Anyway, thems my thoughts. YMMV.
elb