For most self Defense situations in a home, a double barrel shotgun with shorter barrels is about a good a choice as you can get. If the lights are on so the bad guys sees the shotgun, that may end the assault right there. You don't need large buckshot in the gun, either. use light trap loads, because at the range you will shoot in most homes, we are talking 10 yards or less, and a pattern no bigger than a 12 inch softball!
For outdoor work, either a pump or Semi-auto shotgun, or a rifle needs to be the first choice. A handgun that you are intimately familiar with is more likely what you will have when you need a gun. Most experts will tell you that you use a handgun to defend yourself while you move to your long gun. I am a better handgun shot than they are, I guess, because I have never felt I was undergunned with the handguns I use for Self Defense. I like the way they are thinking, but I don't necessarily think its a good idea to suggest to people that a handgun is not adequate for the job. I suspect those experts went to the" spray and pray school of pistol shooting ", and have never learned to hit small targets at 200 yds with a handgun.