While willpower and magic were both intended to be the primary mage stats, it didn't play out very well in practice.
Magic increases your spell power, which is definitely a great thing for mages to have.
Willpower, on the other hand, increases your mana pool and mental resistance. Mental resistance isn't all that important and can be effectively ignored. You might think having a huge mana pool would be a boon to mages, but mana potions are so incredibly cheap to make it's far more efficient to ignore willpower, stock up on mana potions, and set up a really liberal tactic slot for them (say, Self: < 50% Mana → Drink mana potion).
So, you have two options:
If you want to play as BioWare probably intended it, do 2 Magic for every 1 Willpower. By the end game, your spell power will still be pretty high and you'll have a decent size mana pool.
If you want to cheese the game and treat the game as it is, do 2 Magic for every 1 Willpower until you get to ~25 willpower. After that, dump all your points into Magic. Make sure you have someone taking the Herbalism skill, and make mana potions yourself.