Hmm. This has been discussed to death a bit recently, but I'll vote anyway.
Berbatov, because I think he has more to his game. Tevez is mainly about work-rate and urgency, and whilst this is important, Berba has that little bit of skill extra, that moment of class, that can cut open a defence better than Tevez running like a headless chicken, which often is not as useful. Plus, Berba's 1st touch is much, much better than Tevez's, who's 1st touch is somewhat heavy and clumsy.
Still, Berba does have weaknesses to his game, I think we can safely say that. He hasn't turned out to be the lead-the-line striker we all hoped he would be.