For skilling up, you should only be using the Fire Staff. The Frost staff is an important tool for magic users as well, since its superior in single-target situations, and once you hit 50 skill, its DPS skyrockts by 20% by virtue of shaving off 1 Eitr point of its 5 point cast. But the Fire Staff generates way more XP per hit, and does seem to scale with multiple hits, where as the frost staff gains a penalty per hit.

Just to be clear, you should not be religiously trying to skill up your magic skills over 50. You can certainly do it, but it's clear the intent for the ashlands is people to be going in with 40-60 skill, when most assuredly new spells, and spell upgrades will be available.