for that matter, why no talk of 11 gauge - fairly common in the 1700s here
how about an 8 bore..
seriously...i'd lean toward the 12
most post 1780 or 1790 smoothbores were closer to 20 even 28 gauge than 12, but if this is a hunting gun, i recommend using what will be most effective on your game
yes, you can customize the load in the 20, increasing pellet volume to match the 12 with 1 1/4 or even larger loads, but you might experience shot stringing.
stringing is the effect of longer/taller shot columns leaving 'trails'. imagine shooting at a piece of 4 x 8 plywood mounted on a moving car. a shot column that is short and wide will have more pellets hitting the board in one location whereas the same volume of shot in a taller column will tend to string out as it impacts the plywood. think of the extremes of a 2-ounce load in a .410 versus a 10 gauge. which shot column is taller - .410 and it will have a serious string. the difference is less great between 20 and 12 gauge, but still measurable
this is not so much an issue when shooting turkey because your target is or effectively is still, but if you're wingshooting, shot stringing can be important: the longer the shot column, the greater the shot string. a larger bore, all else equal, will have a wider shot column, so less stringing.