MB supposedly follows Satsuki's route, which was never released with any of TM's games (sadly ><). The story is sort of different to allof the routes in Tsukihime, and from what I've played in the story mode, I wouldn't really say you'd be spoiling yourself if you played it. After all, all of the character's killing abilities are known once you fight or use them yourself.
MB and MB Re-Act/Final Tuned both have the same story to them, BUT! Act Cadenza does not have a story mode in it, so if you want to play the story mode of MB (which is pretty short), buy MB or MB Re-Act.
["Here is wisdom. Let him that hath understanding count the number of the beast:
for it is the number of a man; and his number is Six hundred threescore and six."]
--Book of Revelation 13:18