Rondo is a proven NBA Championship winner, and one of the top five PGs in the Association ... Sullinger is a great young player with what looks like great potential, (but is highly unproven, especially in the playoffs, which is a much different beast).
The answer is easily Rondo, and I think that should be obvious to most people who know this game at the pro level, (but I love what Sully has shown us to this point, and am very excited about his future with this team).