Personally, I can't stand U/O shotguns or rifles for one reason only:
I shoot with both eyes open, right eye dominant, and with an U/O the left eye sees the lower barrel and rib (as well as the top barrel), which totally distracts me. With a SxS, the right eye sees the sights, and the left eye sees only one barrel, hopefully pointing at the target!
SxSs rule!