This is a tough one. If they were both in the same contract situation, I would go with Melo. I think they are similar players in that they have the talent to be hall of famers, but they both have holes in their games holding them back from being in that upper echelon. But I think Melo is closer to being a complete player than Rondo, and could be that with the right coach/teammates around him, while Rondo has a long way to go with his shot before he can not be gameplanned away.
However, since they are both a notch below elite, and IMO would need another star (or two)next to them to even sniff a title, I go with the guy who is locked up to a bargain contract over the guy who has one year left on a deal, and will be looking for the max next summer. So I stick with Rondo.