Explanation: when aiming down iron sights, side by side means compensating a little left or right, based on which barrel is shooting. Over-under means aiming once and having 2 shots ready to hit the same location.

Holding an over-under is also easier as it is a single barrel width (perfect handsome) vs the broader handle of the side by side. This makes wielding and aiming the two guns fairly different.

Over-unders are considered preferable for these reasons.