As far as "tactical" shotguns go, the short list is Remington and Benelli. The Remington 870 Magpul Edition is nice, as it offers Magpul furniture, along with a forward pump grip that incorporates mounting points for flashlights and such. The Benelli M4 is a semi-auto shotgun and is the LE/civilian version of the venerable M1014, used by the United States military. The M4 is serious business, cost wise, and they command a price premimum, even used.
If I had to pick between the two, I would probably spring for the Benelli. I like that it offers removable chokes, and if you've ever shot a Benelli shotgun with the ARGO system before, you know you are in for a real treat. However, if I had to choose a shotgun for SHTF, end-of-the-world scenarios, give me the Remington 870. It's pump action (never fails), ubiquitous, and you will always find parts for it.
One last point about the Benelli. There are other versions of the Benelli M4 available, that cut the cost of the shotgun nearly in half. Of course, realize that you are getting a neutered M4, so your mileage may vary.