Before there were O/Us there were SxS...I grew up using a SxS for everything...From deer with buckshot and slugs to rabbits, squirrels, doves, ducks, geese, turkeys, crows and quail, she has done everything...She has a modified and full choke...When quail and dove hunting, I use cheaper low brass shells which helps the modified choke open up quicker...
I prefer double triggers over single, once you get use to them you usually pull the more open barrel first as the game is usually closer...The tighter barrel is used second if you miss...When squirrel hunting, I usually use the full if the squirrel sticks his head around the tree and the modified when they are jumping from limb to limb...
I guess a O/U is OK, but the SxS is the traditional upland game shotgun...No real advantage of one over another as it's what you get use to...