I just went back from an almost 2 month break and had to respec from fire, so i tested frost and arcane.
I'm actually ilvl 483 and in a small guild that just killed Elegon nm.
With simcraft, i sim at 83,5k dps as frost, and 95k as arcane with same gear but spec reforge. My dummies dps self-buff (without timewarp) i went from an average 61k as frost to 70k as arcane.
I haven't been able to compare on all MV boss yet cause i just switched to arcane this week for elegon, but i did 157k dps on our kill yesterday as arcane, topping the meter by a good chunk (35k above the second dps) whereas on all the other fights as frost i was at the level of my mates.
Arcane is the spec i appreciate the least of all 3, but for me there's really a nice boost in dps with it compared to frost. I think frost is competitive, but not optimal seen the gap with arcane.
Both are viable, Arcane does more. Really though I couldn't stand to oplay the no-AB, scorch weaving variant. It's so incredibly dull it drove me back to Frost which I find much more fun, dynamic and versatile.
Thankfully in 5.2 they kill the scorchweaving build so may go back Arcane but to be honest I think I'm having too much fun as frost, and they're adding a few nice changes to it along with a few nerfs (better target switches/burst but less damage overall I think due to the Invo change). I'm hopeing Water Jet or whatever it was called (the water ele move) makes a come back for 5.2 as if I read rightly Freeze won't work on bosses anymore for the FoF proc.
Its fight dependant, some fights frost is better, some arcane. I play arcane for the majority atm. Stuff like Hc stone guards, Hc windlord, and Zorlok i use frost. when we start ambershaper hc i will prolly go frost there as well.
But really they are so close that it should not matter.