I'm gonna buck the trend here, but I think people (citizens, not LEO) worry too much today about over penetration. TO me, its better to have too much, than not enough.
I both agree and disagree. Yes, quite a bit of emphasis is placed on over penetration which, in many cases, can border on sheer paranoia. However, having too much vs not enough? This depends on several variables...none of which I will bother mentioning. Either way, I generally agree.
To the OP-
Given the specific comparison of "effective" defense .357 Magnum loads vs. those of the .45ACP, I vote that, overall, the .45ACP would tend to be less penetrative.
Again, given this particular caliber comparison, I believe that the ability to actually control the weapon should be strongly considered vs. getting hung up on penetration issues. Both of the compared loads are proven man stoppers, penetration aside. Regardless, in terms of control, coupled with stopping capability (and, if one must consider it, less penetration vs. full house .357 mag loads) I think the .45ACP excels.
I adore the .357 magnum...I really do. In particular, I love the 125g. Federal 357B loads. However, such loads are hard to control and from what I gather, can be needlessly penetrative for personal defense use. This is where the .45ACP comes in.
Having said this, I would not get too hung up on the penetration thing as stopping an attack is paramount. And honestly, if a bullet (JHP) does manage to find its way through an individual, what are the odds that it will actually continue on (with enough velocity) to inflict mortal wounds on an innocent bystander? After all, as potent as some of these handgun loads can be, we are not speaking of rifles here. Chances are, if an innocent bystander is mortally wounded, then we are dealing with a complete miss...not a gross penetration issue.
Hence, control/effectiveness first, penetration issues (even though worth considering) are generally second. But, control may not mean as much when firing at, say, 3 yards or less. However, effectiveness will always matter.
Regardless, once
any bullet leaves the bore, risks are involved. One can only assume a calculated risk. This is where practice, maturity and situational awareness come in to play.
Oh...and as a side note, going with a 9mm may not solve all of your problems, unless capacity is your main concern.
A lot of folks DO NOT like the .40 because of the recoil. And as much as I like the .40, a round is only good if you can hit your target.
Too much recoil? Apparently, they have not shot very many large caliber handguns.....
I guess these .40 S&W naysayers simply need more practice and/or should simply quit complaining and stick with an "easier" caliber. Each to their own, I suppose.
Besides, the .40 S&W is a high pressure load...similar to shooting 9mm +P. What do you expect? So ends my off topic rant.