As a couple of others here have said, the best choice for home defense is a shotgun, preferably a pump. When you hear a break-in, grab the shotgun, rack the slide to chamber a round (as loud as you can), and yell, loudly, "Honey call the cops!" Even money if it is an actual break-in, you will hear glass shatter as the perp jumps through a window.
I suggest either #6 or #7 1/2 birdshot as the load, for several reasons. Primarily the fact that it won't go through the wall and kill the kid in the next room, or the neighbor in the next apartment. If you aim at the head, it will stop any attacker in his tracks, and a body shot will do some fairly serious damage, especially at in-house ranges. If you add this Link Removed, which is designed for shotguns, it adds a respectable psychological edge for you. The sight will cost a little over $300, about as much as a decent used shotgun.
For the shotgun, the Remington 870 is hard to beat, as is an Ithaca 37. I recommend the shortest barrel you can find and swap out the buttstock for a pistol grip. That will make it far easier to handle in the confines of your home.
Any pistol round, or heavy shot, can easily penetrate several sheetrock walls and have plenty of energy to kill an innocent bystander, even in another building. A single pellet of 00 buck can penetrate a 4x4 and still have enough energy to kill, as can most pistol calibers.
A shotgun, loaded with birdshot, is also much, much easier to defend in court, which is highly likely if you have to defend yourself.