The theoretical disadvantages of the O/U have already been mentioned. However, as someone who has hunted with O/U shotguns for 40+ years and has spent a lot of time on the skeet field, the larger gape of the O/U is no perceptable drawback.

My experience is that the O/U's are less sensitive to ammunition changes, the barrels do not have to be regulated so that they cross at a specific distance, the line of recoil is identical for both barrels, the stress on the receiver is no greater with the upper barrel than with a S/S and considerably less on the lower barrel. Some shooters cannot shoot well with a S/S shotgun and they would in all probability have the same difficulty with a S/S DR. On the other hand, to many people the S/S DR just "looks" better. Take your choice!