RE: 20ga slug or 12ga slug?
A 20ga sabot slug from a rifled barrel has more energy at 100 yds than a traditional foster slug from a 12 ga smoothbore, and a much flatter trajectory.
For a dedicated slug gun, I would probably go with a 20ga.
I have a benelli nova which I bought as a combo. Being that I wanted the 12ga for pheasant, I have the 12ga rifled cantilever for whitetail.
My 13yo son's 20ga 870 dropped his first whitetail during youth season in september and will likely last him a long time. He can drill a 1/2 gallon milk carton at 100 yds...no deer can survive that.