I am going to level up quests + dungeons, using singular a new mage. which do you think is faster? arcane frost or fire?
What do you mean? I think levelling Frost Mage using "Portal" combat routine have been one of the smoothest levelling experiences I've had thus far in terms of solo levelling, mostly doing quests. That was before 6.0.2/3 so I guess things might have changed somewhat since 5.4. One thing you don't want is levelling as Fire Mage, no "pet" and darn slow spells is a true pain considering Mages are like glass canons, they do plenty damage but can't withstand damage for shit.
frost, just because frost has tools that allows your character to keep the mob away, or handle multiple mobs at once. yea it might be a little slower, but not dieing more then makes up for the time it would of taken to die and run back to your body.
I really, really, really like PLAYING fire ... but frost for afk, hands down. You will need to play with (tweak) the teleport on struck options as you can sometimes blink into more mobs which creates more of a nuisance than assistance.