Your main choice is whether to get a rifled or a smoothbore barrel. Rifled barrels are only for slugs, and at their best with sabot slugs rather than the traditional Forstner type.
Smoothbore slug barrels have a bit more versatility. Forstner type slugs (traditional rifled slugs) are what they work with; sabot slugs won't stabilize without rifling. But they can also be used with buckshot for home defense, and birdshot for close range hunting. They have a cylinder bore with no choke, so 20 to 25 yards is about it for bunnies and such.
Mossberg makes several options in slug barrels.
There are 24" rifled barrels, with the option of rifle sights or what is known as a cantilever scope mount, which places the scope over the receiver but is part of the barrel, so once sighted in, it stays sighted in when the barrel is removed.
In smoothbore, they make a 24" with rifle sights, and an 18.5" with a bead sight. It isn't currently listed on their site, but they used to make an 18.5" with rifle sights. Numrich had some in stock last time I looked, for maybe $100.
I have one of the 18.5" with rifle sights, and one of the 24" rifled with scope mount. The short barrel stays on the gun at home, with the tube full of buckshot. It also groups quite nicely with Federal Truball slugs. The rifled barrel wears an old Weaver K1.5 scope, and is sighted in with Hornady SST slugs. It will shoot three inch three shot (also known as ten dollar groups, sabot slugs are expensive) groups at 100, plus the occasional flier that sabot slugs seem to give.
All that said, I have an uncle who killed more deer than I ever will with his Remington 11-48 with a 28" modified barrel and a bead sight. He used that one gun for quail, turkeys, pheasant, doves, rabbits, deer, foxes, and squirrels. Never found a reason to complain about it.
