It should be Big Baby, since he played better both offensively and defensively than Sheed in game 1 and has been giving much more effort than Sheed for a while. It'd be nice to see the Baby that played so well last year with no KG in the playoffs.
And if Shelden comes off the bench to play with Sheed, I bet people (including Doc) would be greatly surprised at how much better the offense looks. It's painfully obvious, as evidenced by the first half against the Heat, that opposing bigs cheat off Big Baby defensively because they're sure they can block his shot near the rim and they're right to be that confident. It allows teams to shut off anything the C's second unit tries to do in the lane. Shelden's shot isn't as easily blocked, so they'd pay a price for cheating off him down low.
Mike