I'm working on acquiring a 20 gauge NEF single barrel.
I think the youth models with their 22" barrels and short stocks make neat little compact guns that you can afford to use and not regret it. Toss it in the truck, slap a coat of oil on it every couple months, and it will be there when you need it.
They're also pretty light so if you were injured and had to manage with one hand you could probably still use. I actually used to know of an older guy who was a stroke victim who used one for small game hunting. They also come apart with the phillips screwdriver on your multi-tool or you can stick a survivor forend on it if you want a thumbscrew.
I've owned a few 12 gauges before and still do have one (that might get sold before too long), but for my uses anymore I think the 20 gauge is entirely adequate. I live in the NE US and I think a 20 gauge slug will be entirely adequate for any woodland defensive use I can come up with. Regular shotshells are a little lighter than 12 gauge so you can carry a few more for the same weight and bulk but they still have enough effectiveness. The recoil with 12 gauge slugs is also pretty brutal in most single shots. That isn't a huge concern in use but it can be a big deal when you practice. Maybe it's just me, but I'd like to be confident that I can place my shots with decent accuracy and that takes a little practice.
Although I love .22 LR's for their cheap practice and general versatility, I have to admit that it has always been easier for me to get some squirrels and rabbits during hunting season with a shotgun than with a .22 rifle (but I can do it with the rifle too). If you take into account that in a real survival situation you may be injured, cold, fatigued, and starving, I think the extra "forgiveness" built into a shotgun beats a .22 by a little.
I also second the Enfield as an awesome rifle. I'd love to get my hands on a No.4 and some 215 grain JSP bullets to handload for it. They're extremely tough rifles that will work under the worst conditions and can be had pretty cheap, especially if you can find a hack-job sporter to make over.
Although .303 ammo is pretty much limited either to fairly expensive new factory ammo or handloads, that isn't a huge disadvantage IMO. First, if you buy the rifle you should get "enough" ammo with it to get you by for awhile. Second, .303 ammo isn't really that much more expensive than 7.62x51/.308 anymore. Last, .303 is about the easiest cartridge to reload I've ever loaded. Use neck sizing dies only and reloading is even easier than loading straight walled handgun rounds.
For a higher powered rifle I don't think the Enfield gives up anything to a FAL or Garand that it doesn't win back somewhere else and it certainly doesn't lose anything (in practical use) to any of the semi-auto assault rifle knockoffs.