12 Gauge, for most folks.....
Heavier, for the most part.Less kick, harder to lug.
Incredible variety of loads available, from 7/8 oz target loads to 2 oz, 3 1/2" Testosterone Specials.
All else equal(Which they never are in the Real World),more aftermarket goodies, including good stuff like better stocks and pads. More choices.
20s are nice, so are 10s, 16s and 28s. But for the one shotgun shooter, a 12 is the way to go unless there's an overriding reason for the 20. Pixies, tyros, Seniors, etc, may do better with the right 20.