MajorWoods asked some excellent questions !!!
Now I have many slug guns, I do like the 20ga over the 12ga for various reasons: over weight of the gun, less recoil, more accurate, can/will take deer size game including black bear just as well as the 12ga.
Lets talk shotguns: My favorite is my Savage 220f just a damn tack driver, next would be my H&R Ultra Deluxe (single shot) heavy in weight and slightly behind the savage as far as groups. These two would be dedicated slug guns.
If you want a combo type of shotgun that comes with two barrels I would opt for one with a rifled/cantilever barrel, I have a few, I have two Remington's 11-87 in 12 & 20ga, also have a Mossberg 500 combo in 20ga. A great deal of a shotgun. Whatever shotgun you get, get a decent scope I like the 3-9x40, Leupold VX-2.
Ammo: Sabots for rifled barrels, for the 20ga use the 3", for 12ga. use the 2-3/4"