I think they'd lose it equally, but in different ways. Dean wouldn't go as dark as Sam did, willing to sacrifice Bobby. He'd just be reacting as he did in All Hell Breaks Loose.

For me, I feel like considering how Sam seems to show less openly how he feels (unlike Dean who has his emotional moments) and seeing how he broke down after seeing his failure continuously, Dean would likely crack even more. Because the cycle would emphasize his helplessness to change Sam's fate over and over again, as well as show him how he is failing continuously at the one task that he has grown used to doing so much so that it has become automatic: watching out for sammy.

Dean likes to play the tough guy but when it comes to Sam and keeping Sam alive you see his weak side come through.

After losing Jessica, I think Sam has learned to be more tough but Dean has never let anyone get as close to him as Sam. He'd break like a twig every Tuesday.