Well Im no expert...and I dont play one on the net...lol.....although I have played with this shotgun caliber issue a bit. Imo as biased as it is, I think that most people would be better served with a 20 gauge over a 12gauge---primarily because they dont practice enough with the 12. Elderly and others who will not shoot often, or shoot enough to learn to handle the recoil of the 12gauge might continue to practice or be encouraged to shoot more with the 20ga. Ammo selection would be an issue depending upon your environment both inside and outside the home.
As for the type of weapon, any longrifle/shotgun etc., can have retention and manuverability issues within a home, if you tie up both hands in handling a weapon you sacrafice some degree of control if you have to use the phone or a seperate light, turn a door knob, negotiate doorways or hallways.... all of those issues can be eliminated with thought and tactics that will benefit the ensconced homeowner. The dbls that are self cocking (no external hammers aka rabbit ears), will if loaded remained cocked which has been put forth as a possible issue with regards to weakening the firing spring, Im not sure that I agree with that argument, but once loaded and shut the shotgun is cocked which brings the safety up as the next possible issue....the last double I had cut down was an older rabbit ear model......it was in 12 ga. and stout.....----the dbl guns that will be cut down will still get the job done, although they will be slow to reload, depending upon extractors or extruders, they certainly would be intimdating. A pump gun cut down the same would give you 1 or two more shells in a similar package and would be quicker to reload.
Dont know if any of this helps...just my opinions.....fubsy.