The numbers...
The 45 is barely bigger than the nickel, so its not dime vs quarter, but dime vs nickel.
Its been said before... Real world numbers point to there being almost no difference in effectiveness between them.
(I would venture than all pistol calibers between .3 and .45 inch, with a velocity under ~1800fps, are likely to perform roughly the same. Any calibers that deviate from that, are likely, and should be treated as, anomalies in the data... until the caliber and anomaly can be analyzed more thoroughly to verify or refute the difference)
As to effectiveness...
For a physiological stop, all calibers perform the same.
For stopping a determined attacker, it simply remains that a pistol round is basically ineffective unless it hits something important.
If that is the case, the fraction of an inch difference in expanded aize between calibers, does not come into play, except in rare extreme edge cases. The "if only that bullet had hit a quarter inch to the right, the bad guy would have been stopped" cases...
But such cases are not actually significant, not because they are rare, but as it is focusing on the wrong aspect of the situation. (reminds me of the time in WWII the military wanted to add armor to bombers to increase their survival chances. Adding too much weight is bad, so they had to protect only important areas. They looked at planes returning to base to see where they were being hit the most, and put armor there... and it did nothing to improve survivability. Because the areas they armored were not important, as the planes hit in those places obviously returned to base... Its a neat story about a mathematician setting the military straight on how they were looking at the situation wrong... faulty logic)
A miss is a miss, no matter how close.
Saying that a larger round would have been wide enough to turn the miss into a hit is also pointless... As changing the caliber would have changed everything... Lower capacity may mean that shot was never able to be fired (if the shot was like shot 17)... Dynamics of the different pistol, ballistics, and recoil characteristics means that the exact same shot would never happen anyway.
Its attributing significance after the fact, and that is a logical fallacy.
When accuracy is king and proper penetration is queen, then maximizing both is the goal.
So picking a pistol that handles well, is easy to shoot, has high capacity to maximize potential chances at a hit, and one that can be effectively fired quickly... that is the goal.
9mm for most shooters is easier to shoot both accurately and quickly than other common pistol rounds. It has higher capacity than other calibers in similar size pistols, and it has adequate and reliable penetration with good bullet selection.
As far as the standard vs +p debate goes...
Most modern hollowpoint designs get the same and sometimes better penetration from standard pressure rounds compared to +p... From all the tests I have seen, +P is pretty much obsolete in usefulness.
Now in short barrel pistols, it may still be a better choice... many new bullet designs I have seen recently also challenges that niche as well.
I have seen things like the xtreme defender loads from Lehigh defense do very well out of short barrel pistols.
Some loads are +P, but some of the downsides of +P are negated by the lighter bullets. Blast is still higher, but recoil and subsequent wear on the pistol is similar to standard pressure loads.
