Which Shotgun for WTSHTF?

I much recommend a 12 gauge over a 20, unless you are very small-statured. 12 gauge shells are much easier to find (both now and post-TEOTWAWKI), and they pack more of a wallop on the receiving end. There are also a lot of exotic shotshell loadings available (such as CS tear gas) that are only available for 12 gauge.

I prefer pump actions. I would recommend a Remington Model 887 NitroMag pump action. They come with 26" or 28" "bird" length barrels standard from the factory.   The Model 887 pump action,is bomb proof and has special improvements that limit its kick.  Has a great feel to it and additional accessories that come with it to make it a real valuable tool in your arsenal!

In terms of accessories, I recommend that you get:
An 18 inch or 20 inch "riot" length spare barrel, threaded for choke tubes.
A full set of "Rem choke" screw-in choke tubes (including an Extra Full Choke tube for shooting rabbits or perched birds at maximum range)
An Uncle Mikes' brand shell holder (The type with a Velcro closure flap)
An extra long sling (I prefer the M60 padded slings)
Locking quick detachable (QD) sling swivels.

Sling swivels. (TOP mount a QD stud on the stock, and side mount in the swivel in the front so that the shotgun won't flip upside down when carrying it assault style)

Some voluminous pouches to carry spare shells for your basic combat load. (Shot shells are very bulky.)