I can say this much...When I was a kid my single shot .410 put a boatload of rabbit on the table, as well as quail and pheasant. My brother did equally well with his 3-shot pumper. My dads 12 gauge wingmaster got the job done obviously but in the end dead was dead, and based on my recollections of digging pellets out of carcasses, it never seemed to take more than a few.
Dads rabbits were never any "deader" than ours.
I think any advantage he may have had with getting pellets on target due to having more in the air was probably offset by the greater difficulty in controlling the bigger, heavier, harder kicking 12 gauge.
I dont know that I see any real difference in shot placement requirements between the 2 when in an HD situation. On average I don't see the ranges involved allowing for much spread. If you hit it with the 12 odds are you'd hit it with the .410 as well.
It's kind of funny that you see the same argument in the handgun forums. "Which would you rather get shot with, a .22 or a 10mm?" The answer for me is neither! Odds are a hit with either will have me on the floor squealing like a teenage girl at a Justin Bieber concert.
Lots or people are completely happy to use a 9mm or .380 as a HD weapon. Which would you rather get shot with, one of those or a .410 load?