I did the exact same thing Jaga. I played through the game first as a bandit (intended), then wanted to be a mage but ended up automatically becoming a warrior. I figured the most logical place to be a mage was the Monastery, so the sooner I could enter, the better, right?

Wrong. Have you to do Harbor Town first, and side with the Inquisition.

In any case, my 3rd game was a mage, and like Dhruin says, the difference is surprisingly small. The main attack spell of a mage is always crystal magic, which warriors have access to as well, so it's definetly possible to be a spell oriented warrior.