Wow, what a plethora of misinformation. First, if you want a cheap shotgun, get a remington 870 vs. the Mossberg; same price and made better. Second a slug will overpenetrate and go through walls and presents a danger to friends and neighbors. The smaller the shot, the less chance of overpenetration and the less stopping power. You can also stack rounds in the tube: Put the first round in of something like 4 X 6 duplex. A warning and a near-lethal round. Second round should be 0 or 00 buckshot. Still plenty of rounds in a shell, but 000 shot can go through drywall pretty easily.
Problem with shotguns: You have to lead with the barrel. In a CQB (close quarters battle) scenario, ie your home, you have to go around corners. Someone can easily grab the end of a barrel and immobilize you.
.45, 9mm, .40, .41, .357 all provide good stopping power with a well placed shot. The type of ammo you use will determine its penetration. Ball rounds will punch holes in someone and may stop them. You want to concentrate as much energy into the center mass of a target as possible. Their is ammunition out there designed for all the above calibers that will knock anyone on their ass without overpenetrating. You can even use frangible ammo, designed for use in airplanes, nuclear facilities, and other places where overpenetration poses an extreme threat.
Don't forget one other VERY important accessory: A lighting rig. Preferably one integrated onto the fire frame that can quickly be turned on and off. Surefire makes the best in the industry.
Whatever you choose, a course in defensive firearms would be of great benefits.
Also, consider the laws in your state. SOme states are not very sympathetic towards killing people even inside your own home.
If you are going to buy a pistol: GLock, Sig-Sauer, and H & K are IMO the best handguns; S & W, Dan Wesson make great revolvers. Taurus also makes great revolvers, but their autos suck shit.
Hope this helps.