The only question is:
Who is more likely to be the best player in a series? I think either are good enough to carry a team as the player throughout a season, but Melo is probably a little more likely to carry a series for you, because Rondo has a singular weak link in his game, that I don't have confidence in him fixing (shooting, ft%) - of all those PGs, none could be exploited so easily. I fear if he's playing slow or loses a step with age, his peak years will be shorter than an elite wing - so even though Melo is older, he'll have more peak seasons.
Additionally I think you have a shot at picking up an elite PG later - I could see Deron or Paul become free agents or Paul get traded.
BTW - I don't think DEN would want Rondo - they have a very good PG and a young replacement... maybe a three team deal...