[Feenicks]

Magic Attack Bonus is a multiplier - for each point of Magic Attack Bonus, your (unresisted) damage multiplier goes up by 0.01. Meaning that a Magic Attack Bonus of 10 gives you an extra 10% on your nukes.

INT determines your damage relative to the monster's INT.

So if you're fighting a monster with high INT, it's better to have more INT to get a stronger base damage, but if you're fighting a monster with lower INT, you don't need as much INT and can put on more MAB gear to take advantage of multiplier bonuses.