Parker's the better scorer and shooter, Rondo's the better passer, defender and rebounder.
If you need a first, second or third option on offense, Parker is much better for that role. If you already have first, second and third options on offense and need someone to create opportunities for those scorers by penetrating and passing, grabbing boards, getting steals and helping on D, than Rondo's better for the job. Rondo may have a ways to go to become a go to scorer, but he's already better at his role than Parker would ever be.
The risk with Rondo is that he will never develop into a Parker level scorer and thus require Big 3 level scorers surrounding him to thrive. Given the expense levied on scoring ability in the NBA, where even inefficient volume scorers can command top dollar, could make building around Rondo difficult. Thus given the salary cap, it's especially important to sign Rondo for as low a figure as possible to round out the roster with scorers.