Frankly, from your described use of the shotgun I'd get it with a smooth bore Improved Cylinder bore instead of a rifled barrel. Wtih a smooth bore you would have a lot more variety of ammo you can shoot instead of just slugs.
I have a Mossberg 590 with 20 inch Improved Cylinder smooth bore and ghost ring sights. I can put 5 out of 5 slugs on a paper plate at 100 yards with sabot slugs, and 5 out of 5 on the same target using conventional Foster slugs at 75 yards. Certainly not rifle accuracy, but plenty accurate enough to put a whitetail down. With the smooth bore barrel, it'll easily handle any 12 guage round that will fit in the chamber including regular game loads, bird shot, buck shot, plus I've fired special purpose bean bag rounds and rubber rounds.
If you want a rifle accurate short range firearm, go with the rifled barrel. They are certainly dandy for shooting slugs, but if you want the full flexibility of a shotgun I'd recommend a smooth bore.