How can you expect to shoot a O/U as well as a SxS when you can't even see one of the barrels? ;-)
Considering many of the best shooters in the world use O/U, I'm guessing it has something to do with 'dancing with who you brought to the dance'.
Many articles have been written about what shoots best, and if you line up enough folks, you may get a consensus, but I reckon you won't find the answer. I'll be the first to agree, of the shotguns I've shot, I love to shoot a SxS the best - English SxS to be precise, but I know a good part of it is in my head - because it's the gun I was taught to love and respect.
