One of the wisest things ever posted on CR.
I would recommend the Remington 870, vance's has decent deals on them right now. I'd be interested in the deal they have with the 2 barrels for $399. Do yourself a favor and get a good forearm light/flashlight for it. Its a good idea to identify the target, you know.
I've never owned a mossy, but I've shyed away from them on recommendations from friends and forums.
You won't go wrong with an 870.