Them walnut stocks are pretty, wouldn't stay that way more'n a season in the salt marsh. I'm about function, not looks. I buy a shotgun to hunt with, not to look at.
True enough. However, if you want a fitted stock for any reason, the 870 is your best option. If the standard Mossberg stock fits you out of the box, by all means, get the Mossy.
When I said "nice", I didn't just mean "wood replacement parts" like the ones above, or "extra fancy walnut", though of course the availability for the 870 is excellent for both. Composites and custom-fitted stocks are also more readily available.
Depends what you want, and what you want the gun for. Also depends on whether the standard dimensions of off-the-shelf guns work for you. I'm lucky that I can shoot most off-the-shelf guns okay. I know people who can't, and at least basic fitted stocks are a must for them.
Salt marshes are a really harsh environment, but desert quail hunting requires a gun that shoulders instantly and instinctively. I've moved away from 12 Gauge pumps for that kind of upland shooting; too many days coming home empty handed. The guns just don't pop up quick enough. I can see wanting a fitted gun for that, too. I wouldn't use the same gun for waterfowl, though.
An 870 can be made into a winning trap gun, too, with readily available accessories, or a very accurate deer gun (Hastings barrels aren't cheap, but they're good). If you'll never want to do that, hey, I can see that, too. Just like I can't ever see turning a 500 into a muzzleloader.
Bummer about the 1300. That was a nice gun. I wonder if the American Winchester will be resurrected.
BPS deserves a look, too, BTW. Lefties in particular will appreciate that it is ambidextrous, and non-slobs will like picking up their shells from their feet rather than somewhere in the bushes. Works well for lots of things.