They're about the same.
Mandau's better for stronger mobs and Twashtar's better for weaker mobs. Rudra is stronger than Mercy, but you have to offhand the tp bonus dagger. With my gear sets, you have 4409 vs 4426, Mercy and Rudra respectively.
Mandau
-when you gear for Mercy, you gear str, so you potentially have an fStr advantage, and an attack advantage(both from the extra str and the dagger).
-use whatever offhand you want.
-5% crit rate aftermath
Twashtar
-assuming tp bonus dagger, you get a slightly stronger WS, though spillover tp makes it a definitively stronger WS when fStr/atk is already capped
-ODD aftermath procs on additional hits
Not having athos body is a ginormous blow to Twashtar though, so that needs to be kept in mind as well.