Seems to me that the decision comes down to two things (1) performance, and (2) aesthetics.

From a performance standpoint, I would take a close look at what competitive shooters use. In trap, skeet and sporting clays, the top shooters will typically use an O/U (or occasionally a Semi-Auto), but, O/U's dominate the competition. There has to be a reason for that.

Aesthetics? It's hard to beat the graceful lines of a nice SXS. There is a wide variation in aesthetics among O/U's. I use a Browning Citori and have really enjoyed using it in sporting clays competitions, but, fIind that it looks rather obese next to many of the Berettas and some of the other nice Italian O/U's...