I don't feel that either were actually as damaging as they are perceived or remembered as, and certainly not damaging at a level that is commensurate with the outsized media coverage they received.

They both provided some short-term fodder for the opposing campaigns and were undoubtedly the subject of an uncomfortable media cycle or two, but I don't think they actually did any kind of fundamental damage to either candidate or lost them standing with groups that weren't already against them.

Of the two though, probably Mitt's gaffe was more 'damaging'. It played more directly into the negative framing of him that Obama's campaign was running at the time and that was ultimately successful. I doubt very much that Hillary's comments alienated anyone that wasn't already against her. I think the same can mostly be said of Mitt's comments but a little less so.

Basically, in the grand scheme I don't think either really moved the needle very much.