Shrike,as you said its personal,however in American skeet you mount the gun before the bird(clay)is released whereas in international or olympic skeet the gun is held at the hip and is mounted only after the bird has been released.This split second makes all the difference,if the gun does not come to the shoulder properly or a bit late you miss the target.Also with the O/U you have a slight advantage in locating the bird after you have mounted the gun as compared to the S/S.That's probably the reason why all skeet champions use O/U in this sport.