Lovec,
Here's what Hallowell & Co says about the issue:

SxS or Side by Side (barrels) - Side-by-side barrels are better than over & under barrels because they have a broader and more quickly-acquired sighting plane. Although one should always concentrate on the target and not the gun, in field shooting, having subliminal consciousness of the position of the barrels is a real aid to throwing the shot towards a suddenly-appearing moving target. Although not as precise, it is quicker to find the broad crosshairs of a hunting telescopic sight than the fine crosshairs of a target scope. Side-by-side guns are easier to load than over & under guns because the barrels do not need to be opened to as wide an angle (gape) for cartridges to clear the standing breech. Side-by-side guns traditionally have been considered more elegant of line than over & under guns.