It depends on what role you think Robin should play. If it is to be the next Batman-in-training, then it is Tim, no contest. If it is to be what Tim said Robin should be in A Lonely Place of Dying, then it is Stephanie, no contest.
That said, Steph does not have to be Robin to acheive that goal, while Tim does. The moment he stops being Robin he is no longer Batman-in-training.