In a general comparison between the 870 and 500/590's, I believe the Mossbergs have a distinct advantage. The Mossberg's shell stops are designed for easy removal, so never become a problem, where the 870's can break free of their stab locks, and tie up the gun. The Mossberg has an ejector secured by a screw, whereas the Remington's is riveted in place, and is a problem to swap out. The Mossberg safety is located conveniently, and doesn't require movement of the trigger finger to engage or disengage. The Mossberg's action release does not require the shooter to break his firing grip to manipulate.
When I said to a lessor extent, I was referring to local popularity as it pertained to marine shotguns; around here the Mossberg tends to be the runner-up behind the Remington. But if folks really looked at the differences between the two guns, I think most would go with the Mossberg. In a comparison between the two brands, regardless of finish, the split is probably closer to 50/50 as the majority of guns encountered are blued.