Depends on your usage. O/Us tend to give an easier sighting plane for target shooting, and so many prefer to carry that over to when shooting a field gun. SxS guns have different recoil characteristics,where if the gun is not properly fitted' especially cast, then success is minimal compared to the O/U
I love the lines of the SxS, and own two I like to use for birds, as I prefer double triggers for instant choke selection, and I can't afford a used Beretta ASEL in 20 with double triggers. But when it comes to shooting competition targets, theO/Ugets the nod first, and the semi comes in second.
Another reason is also due to weight. Most SxS guns are designed as field guns, so they are light and the recoil from competition or Argentina doves would leave in need of an ortho surgeon for your shoulder