I'm actually fine with them being similar spells. I think as they tier up, they could shift though. Different sizes of AoE perhaps for tier II over tier 1 for starters.

Elemental resistances should actually have more effect on mobs, which I think is a stronger adjustment needing to take place. Looking at past Final Fantasy titles, there is a clear difference between mobs that are strong/weak/neutral to an element. It's hard to see the effects of elements in play currently with the randomness of damage calculations.