If you plan on hunting nothing but grouse get the 20 and use lead, if there is any chance you may want to hunt waterfowl and that includes woodcock, get a 12 because of the steel shot regulations. Most of my hunting years were before the steel shot requirements for migratory birds, licenced first in 65, and the twenty was a more versatile gun then with lead, but now, you need the extra volume of the 12's bore for the less effective lead substitutes.
also: can't go wrong with a Remington 870, with a 26 inch barrel and an assortment of screw in choke tubes, in what ever gauge you choose.