I don't think it matters that much
What game are you hunting. I primarily hunt doves and quail. I hunted for years with a 12 gauge, then dropped down to a 20 gauge, now I hunt with an O/U 28 gauge and I still get more birds than most of the guys I hunt with. A smaller gauge is much lighter, has much less recoil, is a lot more fun and pleasant to shoot, and at the end of the hunt your shoulder is not black and blue and you don't feel worn out. I don't think I give up a thing by using a 28 gauge, or, what little I may give up in killing power is more than offset by the advantages of a smaller gauge, and it is a whole lot more fun to shoot. Heck, I have even shot 'em out of the tree I was sitting under - one handed. For ducks or geese or large birds, you probably need a 12, but for smaller birds, a smaller gauge is just fine, in my opinion. I originally bought the Beretta O/U 20 guage for my daughter, but after I hunted with it, I use the 28 unless we are hunting together.