It depends on your playstyle and your gear. I was frost for early MV, switched to fire when I hit 25%. Only spent a few weeks like this before the nerf. So i'm not in a bad place to be either arcane or frost. Fire has wide swings, if you are ok with that, go fire. I find it a bit boring since the crit mass nerf.
Depending on the fight, I'm either frost or arcane. I do not do hard modes at this time though.