My choice...
If you are buying it for hunting, then I'd suggest the Mossberg 935, because it is a good, tough, easy-to-use, all-purpose shotgun, that can handle some of the most powerful 12-gauge game and fowl loads on the market, because it is built on a 10-gauge frame. If you go this route, then get the synthetic stock with the weighted inserts, to reduce your felt recoil to almost .410-gauge levels.
If you are buying it for law enforcement, then I'd suggest the Mossberg 835, with the (special order only!) 18 1/12" barrel, because it handles more of the specialty rounds that are on the police market, such as the 3 1/2" graphite "door buster" (often, many doors take a second shot from even a 3" magnum shell, to bust the hinges off a door), the 3 1/2" rubber buck / ball (for crowd control), and the 3 1/2" sniper-friendly 1oz. sabot round (which is capable of 3" groups at 100 yards, even with a "cylinder" choke on the end of the barrel).
The reason why I suggested the 935 for hunting, and the 835 for LE use, is because - WHEN YOUR LIFE DEPENDS ON YOUR GUN - you don't wan't a malfunctioning shell in your tube that has to be cleared by removing your shooting hand from the weapon, while you piddle with the action, and re-charge the weapon. If you're just turkey hunting, go with the auto.
IF you can get your hands on an old Franchi-Spas auto-pump, and have a gunsmith in your area who can widen the throat to accept a 3 1/2" shell, then the barrel on that gun can handle the increased pressures of the larger shell, and you'd then have 6, 3 1/2" shells in your shotgun. But, I digress.....
Later,
Stephen W.
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"Vi at tamen honore" (By force, yet with honor) - WYATT