The SxS field gun, especially from some of the better makers like AyA or Arrietta are balanced perfectly for field use, and are light - as in 6# for 20 and 6#12oz for a 12. Their weight and even balance tends to make them feel "whippy" to many. Good for birds juking all over, bad for trying to follow the smooth target line of a clay target - where some forward bias is preferred. The O/U tends top have a raised rib - this helps in keeping the head more erect -again a good thing for clays, and somewhat OK for birds. Most SxS guns tend to have a "swamped" rib, as you are not looking down it, but have your off hand as far forward as possible and you are trying to follow the live bird's movement. As I mentioned before the recoil is also different. An O/U tends to recoil straight back and straight up, whereas the SxS has more lateral yaw to go with the rearward recoil and upward movement. Because their barrels should also have a convergence pint, the fit, especially cast is important on the SxS to make sure the barrels align perfectly for your build -this is not as critical with the O/U.
I am referring to guns similar in build and quality like Browning and Beretta O/U, and the better Spanish SxS.
While sporting does have side events specifically for the SxS -(I won my class using mine at a recent tournament), when it comes time for the main event and the money, the O/U comes out