I have thoughts on this but my thoughts are no better than anyone else's.
I have carried a lot of different handguns over the 46 years that I have owned handguns. I suppose it could be said all were carried "for defense" ---- in that if my life were to have been threatened at the time I was carrying a K22 or a Cold Huntsman, I would have used the 22s for defense. But in all the years I did carry a handgun as a weapon specifically for threats from other humans I have only used 2 calibers. A 45 ACP and a 41 mag.
Any gun is better than no gun, when you need a gun.
A good hit is better than a miss with any caliber, and in most cases a good hit with a 22 pistol is better than a slight wound with a 308 rifle.
It is super common for some men (especially cops) to believe that they are experts on what is a good "defensive gun" because they carried guns for 30 years. But in the largest majority of cases, those ex-cops have never shot anyone, and those that have, have not shot lots of people with lots of different guns, and each one of them used with lots of different loads.
So coming back to my thoughts I would sum thing up this way;
Any gun will do if you can do a good job with that gun.
Bigger bullets are usually better than smaller bullets if velocity is the same.
More velocity is usually better than less velocity.
Bigger bullets going faster are louder and kick harder, so their advantage may be lost in the hands of someone that can't do a perfect job with that combination.
So it's is obvious that a 44 mag with a 240 grain hollow point to the belly is better than a 25 auto with a hollow point to the belly. A 378 Weatherby Mag Mag in the same place is "better" still.
As "gun-guys" we love to compare and debate, but we too soon loose track that it is the skill of the shooter that is far more important than what he shoots.
2 times in my life I have been shot at with 12.7MM Machine guns. I was 100% unhurt. Why? Because I was not hit! All that power didn't hurt me a bit. Wasn't even painful. (it was very scary however)
I would be very fearful of an enemy with a 32 revolver or auto who fired that gun 2000 a year for 5 years. He (or she) is probably going to make good hits. If I was to be fired at by someone that just went out and bought a 32 (or a 9mm, 45 auto, 44 mag, or a 500 Smith and Wesson) and didn't train with it, I would probably be a lot safer than if I were to be shot at by the person armed with the 32, who has fired it 10,000 times.
So if you can shoot a 32 well, and you like to shoot, if you will shoot it a lot, I am sure you will do quite well.
Is that a 100% certain way to win any fight? NO!
There is not such way to fight. All fights are dangerous.
I would always rather be lucky than skilled, but I can't make luck.
I can make skill. So can you.