On the portrait screen it says, if you click on Willpower, "Willpower governs resistance to spell effects........+X modifier is factored into your magical resistance".

My question: If, as an Advantage you take Immunity to Magic, does the size of the modifier make any difference?

I think "Magic" is just one spell flavor....like Fire, Poison, Frost, etc.

So Willpower effects resistance to ALL flavors, including "magic". The chance you'll "save vs spell", that is.