I notice you are now considering shotguns MrP...so I will chime in...after years of seeing things...a Shotgun would be my choice.
For close range and moving targets they are the best...slugs take you out to 100 yards with accuracy...in a SHTF situation they really work well. Targets are hit easier and stay down.
Ask Gravelface about people "not" going down after central mass hits with 5.56mm rounds...I have seen the same...I bet P/K has as well...
I carry a Shotgun as my back up weapon to my Sniper rifle...for use in close engagements or unplanned encounters. Sometimes I am limited to what I can get issued. If I had my choice or was working privately I would use a Bennelli M90 or I like the Saiga with seperate Mag feed.
Above all else though...only buy what you can practise with. Guns in cupboards are not much good. If a pistol range is round the corner and a shotgun range is the outer city limits...save your money on the shotgun and buy ammo to practise with the pistol.
A master of one weapon beats a collector of two...all the time. <img src="/ubbthreads/images/graemlins/thumbup.gif" alt="" />