The answer to the question is "Yes, No and Maybe"
I don't think you can make the statement broadly about all guns. I expect that the 6.5lb sxs that I use to hunt grouse won't stand up to the same pounding as the 8.5lb over/under that I use to shoot targets. Conversely, a Winchester Model 21 sxs would probably exceed the longevity of light English over/under.
Over the long run, however, I'd have to believe the over/under would have the edge in durability because the barrels sit in the frame whereas the sxs barrels sit on top the frame. The recoil forces on a sxs are presented laterally as opposed to vertically with the over/under. Over the years, sxs makers have developed enhanced locking mechanisms and other tricks such as sideclips to reduce the side to side forces of recoil.
I've seen many loose sxs guns over the years but very few loose over/unders. Based on my own experience, I'd give the durability nod between the two to the over/under.