i have been both frost & fire at lvl 70
i would say about 15% more damage (per second) for fire
if you use scorch 5x probably a third more dps
fire criticals are a little higher but are a lot meaner due to ignite in terms of total damage
frost does more damage per second against frozen targets, does more damage per second with a water elemental and does more damage per second with icy veins, probably in all 3 cases more than a fire mage does but a fire mage can sustain more damage for a long time and likely overall do more damage
still do not underestimate the frost mage