I can't believe I'm the first to reply. This is an ancient debate.
Flip a coin.
Okay, seriously? I think the 870 has an edge in durability and ease-of-accessorizing (it simply goes with EVERYTHING, darling.
) The former is certainly debatable, the latter is certainly minimal. Cost? The Mossy is a little less expensive, I think.
I'd try them out and see which you like best. The differences are small enough that 'personal ergonomics' will likely be your deciding factor. For instance, some people love the Mossy tang safety. others can't abide it.
For the record, the 870 is THE benchmark by which other pump guns are measured. It got to be that way for a reason. But it is truly telling when even the most die-hard supporter of one gun will readily admit that the other one is perfectly acceptable as well.
Mike
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"A human being should be able to change a diaper, plan an invasion, butcher a hog, conn a ship, design a building, write a sonnet, balance accounts, build a wall, set a bone, comfort the dying, take orders, give orders, cooperate, act alone, solve equations, analyze a new problem, pitch manure, program a computer, cook a tasty meal, fight efficiently, die gallantly. Specialization is for insects." -Robert Heinlein