Well, I'm gonna have to agree with Taerdin, because in all the sites I've checked out they've described intelligence as the "magic attack" and spirit as "magic defence". That is the only site that I've seen that describes them as the stats that determine the power of white magic or black magic.
I would also have to agree that Mithras rock. Though I haven't played the game, I've noticed from all the places I've checked out that if you want to go physical, go Mithra or Galka, but if you want magic go tarutaru. Though all of this is solely based on base stats.