Stats are the same, it's not dumb like FF11 where everything had different modifiers from everything else. INT and Magic attack potency will boost CNJ/WHM nukes just like they do for THM. There's very little if any overlap of INT/M Attack so I don't think you will have to pick between one or the other.

Concerning Elemental Potency vs. Magic Attack: Magic Attack is general purpose but Elemental Potency comes in larger numbers. It's up to you but most people will go for the Magic Attack.

WHM is really good at nuking, the problem really is that two of the three most prominent slots for magic attack potency or INT, Head and Feet, are most likely going to be taken up by the REALLY GOOD AF pieces available in those slots. You could probably get away with using an INT headpiece instead of the AF head, but that Regen boost on boots is just too good.