I'n thinking Mage myself. If nothing else, his bonus skills seems to give him the most dramatic advantage compared to the others... I mean, Higher Magic gives 2 spells per turn. It practically doubles his firepower.
I do worry about the lack of Rage buildup with the mage though. Even with mostly ranged units, in most fights in the demo my units barely managed to hit once before the enemy died. I makes for very little rage, I am worried that I won't get to see all the rage skills as a mage. The rage experience skill will propably have to be maxed ASAP. Perhaps I'd even have to use Order magic instead of Chaos to make the battles last longer... but where is the fun in that?
Still, I'd say the mage is worth it just for the fact that he seems to have the fastest gameplay of the tree. In the demo I finished most battles with him in half the time.