I do agree with SigFeind's assessment that just about any wound trauma will PROBABLY stop an agressor whose sole motivation is monetary or material gain. Such individuals are PROBABLY not motivated enough to continue agressive action once they have been seriously wounded, or even face the threat of serious bodily injury. If they were, they would probably not be criminals and have real jobs.
However, I personally don't like to take chances. The problem with the above statement is that the actions of a person once engaged in a lethal force encounter are VERY unpredictable. If the unthinkable happens, and I have to pull the trigger, I want the cessation of hostilities to be involuntary and rapid. For a determined agressor, this requires a permanent crush cavity that penetrates deeply enough to reach vital cardio-vascular structure. I believe #4 buck is the minimum pelet size that meets this requirement. Shotgunworld.com • View topic - 12 gauge Wound Profiles (56k beware) Examples on page 3. I believe #1 buck provides the best combination of permanent crush cavity penetration depth, volume, and cross-sectional area.
Personally, I prefer my 20 ga because if fits me well, and swings and points faster than my 12 Ga. I load standard #3 buck, and keep my Walther P99 in .40 in a holster next to the bed for backup (along with tac-light, cell, and 4" folder). I will concede that I need some specific SG training (any recommendation would be greatly appreciated).
IMHO, The best back-stop between you and your neighbor is the torso of the perp. If you aim at across the room distance, overpenetration should be a non-issue, unless you're using slugs.