Depends on whether it's a boxlock or sidelock as far as strength, the boxlock being inherently stronger by design. I'm not aware of any O/U sidelocks so i'd say as a rule the O/U is stronger though if compared with a boxlock SxS then thing are equal, and personal choice comes into play. I prefer the lines of classic SxS double in the larger NE chamberings, and they are IMO easier to reload in a high stress situation with both chambers being on a equal plain. But for a non DG rifle a O/U double is dandy a Chapuis in 9.3 x 74R for example being very appealing to me.