Mind is related to the power of white magic (curing esp.) and magic defense. If you've ever resisted a spell, it's because of mind. It also determines the potency of a cure spell. However, cure spells have upper and lower caps on their effectiveness. Cure 2, for example, will never exceed 90 points, except when boosted with a Divine Seal (or, so I just heard, on a certain day of the week when light spells are boosted). Under normal conditions, once a spell hits a certain effectiveness, boosting mind further is futile.

Intelligence is related to black magic damage. The higher one's intelligence, the more a spell will hurt the target. It also effects whether a spell is resisted. A low int mage casting fire on an enemy might find his spell dealing 0 damage or being resisted entirely. If you want your enfeebles to "stick", as they say, int is the stat you need to raise. EDIT: A white mage wil find high intelligence useful as well, because the Holy spells and enfeebles are affected by this stat.

That said, even more important is simply raising the stat for the particular school of magic. Raising your "Healing" ability is more directly beneficial than raising mind the same number of points, in terms of healing capability.