I have both and both do well. I personally prefer the classic looks of a nice SxS. My son shoots my Superposed O/U more than I do and he is very good with it. I let him shoot my SxS and he was missing a lot. Must be something about the wide vs. narrow sightplane because they are the same length barrel, same choke, etc... You should try both before you decide.

You can usually get a good SxS for less money than a good O/U.