Terra has the better story and character arc. While I still enjoy Celes immensely and she is certainly attached to my favorite moment in the game, reading background notes about how her story was meant to be made me feel she was poorly utilized and only got upgraded to major character status due to Kitase taking a liking to her. Mechanically, they are the same, though Morph/Trance is a much more useful move than Runic, that's largely made moot by game mechanics anyways, so they are still roughly the same.