HD, shotgun or handgun.
I've got a 18" barrel pistol grip mossburg, which I bought intending for home defence. I keep the first shell loaded birdshot, with two 00 shells behind it in the tube, a handfull on the shelf where I believe I could grab them. I know you can load pellet shot into pistols, but in my experience, this is almost worthless beyond 8 or 10 feet, my purpose of loading a shell of birdshot is that even from the hip, I'm almost assured a hit on anything I fire at, whatever I hit is going to know it was hit but is probably not dead but hurt. I know my next shot is a highpower 12ga round thats going to floor whatever I hit, and that I'm almost assured a second hit.
My concern with home defence is that I'm able to stop a home intrusion, protecting myself and not hurt any neighbors or innocents, perhaps forcing the intruders to flee without shooting.
I keep the gun unloaded for safety and for the chance that just hearing me rack a shell might scare someone off, that and yelling and most intruders would perhaps flee. It's important to note that I think this is also critical to asses the intruders intent and approach, if they think an armed homeowner is aware of their presence and will not run, this is NOT an opportunist, or pinkish teenager.
Right out my bedroom door is a hall, from the end of which I can see my front door and also hit a light switch that would illuminate the living room. My basic notion is simply that if I've really got to fight someone off, thats as far into my house as I would get, the end of that hall. From the hall I could see anyone going threw the front door.
I know it's simple but I can't imagine what else I might do in the even of a break in. Things could get complicated, more than one assailant, breaking in threw a back window, other things, but as far as plans for the unforeseen go, I tend to think simple is better.
In review: Benefits of the shottie are 1) Loud, unmistakable sound of racking shells, may scare intruders off, no rounds fired, to me the best senario 2) Loading of shots that will stop or deter an assailant (birdshot, beanbag), but will also not carry much power threw neighbors doors or walls, if at all. Also will allow loading of shots that will drop a 200lb boar on pcp (high power 00, slugs).
If you have no innocents to worry about (who?) you could even load slugs with a birdshot or even beanbag starter.
Shotguns offer alot of flexibility, "entree" or "cruiser" style guns are very compact and maneuverable, and they offer *IMO* the best of non lethal and TOTALY letal options.
My choice for HD, the PG 12ga pump shottie.