The reason I would choose the 20 over the 12 is simply that IMO the 20 will kill a deer just as dead as a 12 without all that unnecessary recoil. Both have pretty much the same velocity and trajectory. Albeit the 12 has a bit more energy. But not so much as to make a difference on a whitetail. Not being recoil shy but I know what its like pattering a shotgun for turkey hunting. So I'm pretty sure sighting in a scope would not be a very enjoyable experience either.
So if I were to buy a 20 ga 220 I would get a one piece Weaver style base and rings and either a Leupold Grand Slam scope or a Nikon Prostaff in a mid-range vaiable like a 2-7 or 3-9