I shoot both, it really comes down to preference. The supposed advantage to the O/U is a narrow sight plane and less felt recoil due to a SXS torquing when firing. The other issue with a SXS as a target gun comes down to weight, very few SXS were built with the purpose of breaking targets, as field guns are made lighter to get on target faster in the field. As such, using them for target duty tended to beat up the guy behind the gun more, than a heavier target oriented O/U did, people eventually favored the O/U for it's less felt recoil. This is marketing takes over, remember skeet started out as a practise game for hunters, which is why most of the early champions won using SXS and Pumps, that's what they shot in the field. When the gun companies started making specialized guns for this purpose, then preferences changed towards the O/U.