I'm partial to 870s.
So are all the older, richer guys I know, guys who have decades of experience with shotguns and more 12 Gauge pump guns than I do guns total, and who can and do buy whatever they want, new and old. When it comes to pump guns, they seem to like Model 12s and 870s, with a few Ithacas thrown in. Since the Model 12 hasn't been made since gasoline was .25 a gallon, that leaves the 870 and the Ithaca 37.
I know a guy who has shot a Mossberg to death, just hunting. I know others who have put many more rounds through 870s that still look and work like new. For your purposes, this may not matter, but ease of maintenance does.
The only downside to the 870 is that its structurally-superior receiver can develop surface rust and pitting if poorly maintained and stored, since it's steel rather than aluminum. The Marine Magnum takes care of that.
Or... This gun is Parkerized. It comes with a pistol grip and an 8-round magazine. That doesn't mean a mag
extension, it means a true one-piece 8-round magazine, solidly supported at the muzzle end. Once you're in the over-$500 price range for a specialized gun, it's well worth a look. Smoothest pump ever made, according to some. And since it ejects through the bottom, it won't get oil on the inside of your gun case.
http://www.ithacagunsusa.com/lawenforcementmodel.html
EDIT: Unfortunately, you may have a hard time actually BUYING the Ithaca until they get their lines rolling along later in the year. I'm basing this on another thread I started because I'm toying with the idea of getting an Ithaca for upland birds.