I would go for a Remington 870. I own a modified 870 and a Benelli M3, and I end up shooting the 870 more. You got a lot of accessory choices with with the 870, plus there are many custom shops as well.
Right now Big 5 has a rem 870 security shotgun on sale for a little over $200. Great buy I was in there yesterday and did a double take at the price tag. I guess big 5 is only in the west though. Also dont forget to look at the ithaca 37 homeland security model. You can get them new for around or under $200. They are recently back up and running at Ithaca so it might be difficult to find one at the moment.