I do not think it is as simple as people make it out to be. I went with a mandau thief friend to try out our numbers on some trolls. I was consistently outdamaging his mercy stroke with my rudra's storm by about 1k each time. Perhaps part of the difference is that I off-hand a TP bonus magian dagger and only go when I have 200tp. MS from my understanding gets no damage bonus from more TP.

I would say if your playstyle is to WS as soon as you hit 100, mandau is for you. However, if you like the idea of more consistent one-hit damage, Twashtar should not be ignored.