First, I like the SxS because of tradition and apperance. But that sort of thing is in the eye of the beholder.

The SxS is a tad easier to reload in a hurry. The OU is said to be a bit easier to regulate.

The largest caliber OU I have seen was in 9.3x74R. This rifle was so light I would have been afraid of the recoil. Put the lie to the theory that doubles are inherently heavier than magazine rifles. I have seen several OU's in the smaller calibers, including a Winchester. Up to that point I didn't know Winchester ever sold double rifles.

I have to think the sighting plane arguments apply to shotguns.