A healthy Rondo is obviously more integral to the team's success.
I don't know if it's that obvious. Yes, Rondo is more valuable if you had to choose which one would play. However, if your choice is "healthy Shaq + 65% Rondo" or "100% Rondo + no Shaq", I think I probably lean toward the former.
As much pain as he's in, Rondo has actually looked pretty decent the past two games. He's clearly limited, but he's still a pretty good player. If you couple that player with the Shaq we saw earlier in the year -- when the team was dominant -- I think you'd actually have a more dangerous team.
Shaq inside would help our offense, rebounding and depth, and would make us less of a jump-shooting team. We'd have somebody to punish the weak Heat defenders inside. A limited Rondo wouldn't be as good at making touch passes, but he could still get to the hoop and score.
That being the case, I think I'd go with Shaq.