forums.d2jsp dot org/topic.php?t=54233648
HERE IS SOME TL;DR
Treachery is simply better than Fortitude. In ~80% of all cases when raw damage is applied, and better in ALL cases in which CB is applied. In the eventuality of killing a mob, Treachery kills faster. It also makes for a better shell for the mercenary to be a stronger tank.
Not only do you get that +Res and DR when fade procs but you get IAS and FHR (if I remember well)