I'd take Werdum over almost any heavyweight right now. I wouldn't be able to pick if/when he faces Cain, JDS, Cormier and Barnett.
As for Overeem, I don't think he'll beat Mir. He'll fade and be cut after the fight. And he could be the next Bellator heavyweight champion.
Werdum's offensive striking is improving but his defense still has holes and he still backs up in a straight line when attacked. The Nog fight showed those holes too IMO. I think JDS will remain a terrible stylistic matchup for him. But the Cain fight would be fireworks.