Far and away it's Rondo.
How many playoff games has Sully won us? None.
How many playoff games has Rondo won us? Probably five or six, all by himself.
Look, I love Sully, but to do anything substantial in the playoffs (beating Miami), we're going to need a star, and Rondo is that star. We also need a guy that Miami doesn't really have an answer for, and that's Rondo, too.
Finally, Rondo is going to make everyone else around him better much more than Sully does. Sully's rebounds are massive, but Rondo's ability to run an offense is much more valuable, especially in the playoffs.
This shouldn't even be a question.