One of the distinct advantages to an O/U over an SxS, however slight, is that since the barrels are vertically alligned, they track the same. Side by Side barrels are actually regulated to cross in front of the bead at a given range (25-50yrds for most models), but ultimately, the shooter is NOT looking down the centerline of the bore. For O/U's, the barrels are regulated vertically again at a specific range, however, they won't track left to right like a SxS will, since the shooter is actually looking down the centerline of the bore.
You only usually notice this "crossing" when you're shooting slugs. I installed .45-70 sleeves in a SxS that was regulated at 25yrds. At 200yrds, the left barrel was shooting over an inch to the right, and the right barrel was hitting about an inch to the left of the POA. Each barrel would shoot approx 1.5-2" groups, but the groups were distinctly off center.
Of course, 2" for a shot pattern isn't really critical. And of course, with the O/U, I'd expect vertical stringing too.
Beyond that, O/U's are generally considered to be better balanced, and more stable for fast action clay shooting.
Personally, I like both. I'm more of a fan of the style of a SxS and I knock down a lot of birds with them each year, but I break more clays, for some unknown reason, with my O/U's (honestly may simply be practice).