Always used to be a 12ga shooter, but for the past 10 years or so, its been a 20. The 20ga is lighter to carry, kicks less, and balances well. I have found the 20 is just as deadly for upland birds over a dog as the 12. Now if we are talking waterfowl, or slug hunting for big game, then the 12 wins out - more pellets means a denser long range pattern. Bigger, heavier slug means quiucker knock down on furred game. My favorite combo is a light Franchi 20 over-under for upland, and a 12ga Remington auto for ducks/geese/gobblers, and slugs. Although I use a 12ga for home defense (only because in addition to birdshot, I have a free supply of slugs and buckshot), the 20ga is also fine in this role.
Larry