From Kaeko's blog
Point for point, MATK > INT for every spell in the game except Thundara on Combo (for reasons I won't get into here)
Point for point, INT > MATK by FAR when using Thundara on Combo
MATK > INT still for higher tier spells, but the difference is not as big for upper tier spells like Thundaga
Thundara on Combo becomes more efficient the less damage spells deal (for whatever reason, whether it is dlvl or resistance). Examples of this are Ifrit and Chimera.
INT, MATK, elemental potency are the only 3 "player controlled" stats that affect your non-critical damage with the exception of Holy, which has an additional MND modifier.
There is a MATK cap. You will never hit it on anything you care about.