you should have her in a bunker if you use dark magic too. the beauty of dark magic is that all spells are very cheap, meaning you can cast all five of them every turn. that means 5 extra PD from the dagger each turn. you can overwhelm even dwarf antimagic that way, and you do not risk unnecessary miscasts. Bottom line is: the dagger scales with the number of spells you actually cast, and since dark magic allows you to cast the highest number of spells, it is the school that scales best with the dagger.
that being said, both are fine choices. i prefer dark magic because of the versatility, but you really wont go wrong with either
