[Morgeruat]

the big question is do you want a mage that's prepared for the dangers and most contingencies, ie if you take a mage from the surface you can have him learn all the spells you want, whereas if you take Imoen you'll have to either be satisfied with what she has and what you can pack on your back, or wait for the good spells until such a time as you can use them. Edwin also has more lower level spells if that makes a diference.