Some talented shooters have recorded a number of fine performances at the very highest level with side-by-side guns over the years, but the fact that they almost invariably go back to a over-under should tell us something.
Fact of the matter is that the stacked barrels of an over-under give an edge when it comes to viewing certain clay targets.
The difference might only be two or three birds per hundred but the margin is there and on it rests your chances of winning.
You could say that the heavier weight of an over-under also helps make a difference but this can be cancelled out if the side-by-side user decides to use a gun of similar weight.
Regardless of weight however, where it might cede another vital target or two is through muzzle flip from the first shot.
An over-under tends to be steadier in this respect, allowing the user to acquire his second target a little better especially on those combinations where it helps to keep the gun in the shoulder between shots.