If you're looking for a personal defense AND survival gun, the only gauge you should consider is 12. Ammo is presently more available with more options, and given any sort of shortage from a local run or ban to the end of the world, is the most likely you are to find. Mad Max didn't tote a 20 GA double barrel.
I've never been a fan of the dedicated combat shotgun for home defense. I'm not planning on doing any offensive CQB or door breaching in any circumstance, and I don't feel the need to look cool at the range. Everybody already knows I'm cool, or if they don't they will after I fire a few rounds. I DO like a rock solid and well proven design with a relatively short barrel, an open choke and few snaggy accessories tacked on. A "bird" gun with a 21-24" barrel in the hands of a skilled shooter is every bit as deadly as the most tacticool black shotgun out there, and much better for crawling into the brambles and filling a stew pot with rabbit. The 2 designs presently available as new arms would be the Mossberg 500 and Rem 870. I personally prefer the 870, but there's nothing wrong with a Mossy. Both are stupid popular and common among the general population and LE, and well supported with OEM and aftermarket accessories. A Win 12 or Ithaca M37 could also be considered. If you want a mag tube extension and light rail that clamps onto said extension (or is an integral part of it), they're out there. Just don't get a snaggy one. Duct tape and a contact flashlight has served many well. Smooth is fast, fast is lethal. For this reason, I absolutely prefer a naked bead barrel over a vent rib for such work, and will shun any sort of rifle sight or optic on such a gun. You point the shotgun son. You should be able to close your eyes, shoulder the gun, and hit a man sized target at 20 yards. If you can't, practice more.
Interchangeable choke tubes are a bonus. If you find an old fixed choke gun, find one with IC choke, an old cylinder bore slug barrel, or have a tighter choke reamed out to IC by your local gunsmith.
For loads, I follow the smaller pellet theory. I stoke my scattergun with #4 Naked (not buffered) buck. Most of these you find are made in Europe. I think my most recent ones are S&B. Plastic shot buffer is bad JuJu fired in a confined space. Learned that the hard way in a large pit goose blind firing buffered bismuth and spending the better part of the best shooting time working plastic bits out of my left eye while my buddies filled their bags with honkers. Standard 2.75" shells, as we have a couple of oldies in the fleet also and I don't need more recoil or stopping power. I keep 00 and foster slugs at the hunting camp as black bears are my most likely threat. Find some of the cheapest BB shot steel shells you can for practice shooting. The blast and recoil will be quite similar to full power buck or slugs, although the pattern will be tighter than with #4 naked buck generally.
