someone mentioned how the 12ga has too much recoil. If it does, for you, don't use it. Recoil doesn't matter as much as attitude and determination.
I'm short....there are enough of my posts metioning my height. I have hunted / shot my Mossy 835 for years and have run plenty of 3 1/2" shells through it. And I have seen several "men" that were a fair bit larger than me wuss out after firing 3" steel shot shells. Attitude and determination count for a lot.
Speaking of which, if your opponent sees you nervous on your end of the gun they may well think you don't have it in you to use it. If they see that you mean it, a 22 will ususally be enough to disuade them from persuing their chosen course of action.
As 44AMP said, there are two differing situations here, 911 can / will respond and 911?????: WHAT"S THAT????
If you are carrying a long arm, there is no hiding it. In the "If you are on your own" scenario; deterent force is or can be a major factor....think L.A. riots (BTDT). In an urban setting, a shotgun is not a handicap. If the sight of one is enough to make bad people change directions, it did its job. If they don't change direction, you can change it for them and the followers of Mr. Tough Guy will need to make a moral check (with fail as a leading result). Now that you have proven resolve and determination, they will look for a softer target.
Slow reload?? Ever practiced tossing one in through the ejection port? Did that plenty of times duck / goose hunting. Not as fast as a mag change on your Glock, but it works well enough. Even then, you can top off a partial mag on a pump easy enough w/o taking it off the shoulder if you practice it.
One must consider their individual situatrion. I have a good friend who id absolutely lousy with hand guns....he knows it. But with a 30 carbine or Camp 9 he is well armed. In our setting, a full power rifle is overkill, but if that is what is at hand we can make it work to our advantage. But I sure won't use one if someone is banging on my door at O'dark:30. Same if they aqre coming through the window. Here, properties are 40 - 50 foot wide. Shotguns w/ buck shot, light/intermediate rifles with the right ammo, and standard hand guns are in order. YMMV