You'll also note that some espers teach spells at a faster rate, so with some strategic esper-swapping you can get certain spells faster. This is, of course, if you are not concerned with stat boosts in the beginning. I tend to be far more concerned with getting the spells than the stat bonuses the espers might grant until, as TheAbominatrix said, I'm leveling up, so using the espers that teach spells the fastest is the way to go.