From what little understanding I have of BLM, M Attack is more powerful than INT in most cases. However, for BLM Thundara is generally their highest damage/most reliable spell and it apparently has an INT bonus modifier when combo'd that makes it pull ahead a ton if you gear heavily for INT.

So I'd wager a guess that M. Attack should be what you will want to focus on but as far as I know on CNJ/WHM you really don't have many gear slots where the two stats conflict with one another.