I think it depends how bad the marriage is and the quality of any subsequent relationships that the parents have.
If I am completely honest with myself I do feel that DSS suffered more as a result of his parents' split (12 years ago, and nothing to do with me!) than he would have had they stayed together. DH left because he couldn't seem to find a way past the daily arguments. His own parents had what he perceived to be a cold and loveless marriage and, at the time, it seemed to him that the only way he could prevent his son growing up in the same environment was to leave.
However, it didn't exactly work out that way, as both parents took their issues into their subsequent relationships. Mum still screams and shouts when she doesn't get her own way, just at her boyfriends now. Dad took his head-in-the-sand, emotionally-stunted avoidant behaviors into his next long-term relationship (me!)
I was tempted to walk away too (and did for a while), but didn't want our DC to go through what DSS had, or for DSS to witness yet another in a long line of failed relationships, so agreed to try counselling with DH. I really didn't expect it to work, but he emerged a changed man.
Not every relationship can be fixed and if there is violence or abuse, then run don't walk. However, I don't think it is a simple yes or no answer to whether children are more damaged with parents together or apart, as I do think that DSS would have been happier if his parents had stayed in their unhappy relationship together.
My one hope is that seeing DH and I fix our relationship will help him break the cycle, but he's nearly grown up now and I fear it may be too late for that.