My advice would be to use the 870 Police Magnum. When selecting a HD weapon, its best to choose one which will work no matter what, even if loaded with compressed ammo and covered with dust and fuzz from laying under the bed. For this very reason, revolvers and pump shotguns are excellent choices. Simplicity in this case is a virtue. I also advocate the use of reduced recoil buckshot for those who can use such a load for thier application and typically, there isn't enough charge to reliably cycle an automatic shotgun. The reduced recoil loads have many virtues and its not practical to have a weapon which excludes thier use. Another point to mention is huge problem with a fighting shotgun:ammo capacity/consumption/reloading. A typical HD shotgun will hold between 5-9 shots, which really isn't that much considering all the possbilities you may be faced with should you have to deploy your weapon. Automatic shotguns and pistols have a tendency to be shot dry rapidly during engagements with nothing to show for all the expended ammo. A pump shotgun forces more deliberate and conservative shooting. Its not possible to spray an area and
empty your weapon before the first case hits the ground as is the case with the Benelli. Reloading your weapon isn't as easy as realoding a magazine fed weapon which makes ammo conservation that much more important in a HD scenario....maybe not so much consumption as economy. Your rounds really need to count. You cannot afford to empty your shotgun in a HD situation without making a neutralizing hit...While the Benelli M1S90 is an excellent fighting shotgun, but nothing can top the simplicity and reliability of the 870, which is the ultimate choice for a defensive arm.
Also consider the psychological impact of chambering a round, this may be all it takes to defeat your opponent.