The OP is carefully weighing options, and reasoning through what would likely be best. That's a good way to go at it.
Bartholomew Roberts gave us a good dissertation on the effects of birdshot, possibly drawing from this video.
http://www.youtube.com/watch?v=yIfilArIHlY It's worth your time to watch. It will give more perspective than anything any of us have to say.
Most people who are shot with handguns survive. The best stats I can find say that about 6 out of 7 live.
If you are attacked, and pull out a firearm, the best stats I can find say that 93% of the time your attacker will flee. So only 7% of the time does it even matter much what kind of firearm you have.
I wouldn't be too hasty to apply the 12" rule for metallic cartridges to shotguns. The mechanisms involved are quite different.
Statistically, few people who are shot at close range by a shotgun survive. I had the actual percentage, but it has escaped me. The bottom line is that at close range, shotguns are much more deadly than handguns, as a general rule.
Number 8 shot is probably the least desirable shotgun load, and even that will punch a hole the size of a man's fist, 4-5" deep, in an attacker's abdomen at across-the-room ranges. Such a wound will get your attacker thinking about something besides harming you. He's going to ground, and is not getting up. Graduate to #4 to #6 pheasant shot, and it's much more effective. (#6 will blow the snot out of two layers of sheetrock, as in an apartment wall. I had a friend who experimentally determined this.)
Would I rather have #4 buck? Or 00? You bet. There is no doubt that they are better than birdshot. Even at 30 yards, those are devastating loads. But at close range, even lowly birdshot is more protection than some people realize.