It really just comes down to personal preference. I finished Risen as a mage with level 9 Frost and level 6-7 Fireball.
Once frost gets high enough, it does a fair amount of damage and also freezes your opponent for a short time, during which you can pound on them with melee weapons.
Just remember that you won't have anough learning points to develop all of them. Pick a main one to concentrate on, and then you can add points to a second one later if you want.