I say Yi. Bogut has developed nicely into a double-double type of center who can pass very efficiently. But Yi has better offensive moves and more athleticism. Defensively, Bogut has the bulk but Yi is quicker.
Bogut is slightly better at the moment and he has been more consistent than Yi. The Bucks game against the Bobcats has shown what Yi is capable of though so Yi will be better in the future, maybe as soon as the end of this season?