Every way you slice it, the Mossberg is a better gun than the 870...speaking of combat shotguns. As sporting shotguns, I think most anyone would rather have the Remington, and I wouldn't blame them. As wonderful as the 870 is, it is inflicted with a critical flaw for a combat shotgun, the short stroke that will lock the action and take the gun out of action. The Mossberg WILL NOT do that, because it's basically a simplified/somewhat improved copy of the High Standard Flight King...which is itself, a copy of the old Remington M31; a shotgun Remington should have never discontinued. And the HSFK/Rem31's action is BY FAR the smoothest functioning pump shotgun I've ever seen, and they are absolutely, utterly reliable. So Mossberg was very wise when they chose their starting point for the 500 series.
The 500 series took the M31/HSFK action and modified the locking mechanism in a way that would accommodate a removable barrel. In doing so, they completely killed the smoothness of the action, but gained the removable barrel feature; and of course, kept all the wonderful reliability.
Funny, but Remington made the model 31 when they abandoned another amazing design, the M17 which Ithaca then took and made the model 37.
Now from a sales standpoint, one can never fault Remington for dumping two of the greatest pump shotguns ever made, because the 870 went on to be the best selling shotgun in the history of firearms; and it's hard to fault that. And as a bird gun, the 870 is just outstanding.
But from a technical standpoint, both the M17 and M31 are just leaps and bounds better than the 870.
Me personally, for a combat shotgun I'm a Winchester '97 kind of guy...who is currently Winchester '97-less. So filling in is a 20 bore Remington Model 11; another wonderful classic.