Completely depends on the mage aswell, saying arcane is better than fire on all fights is a silly statement, it generally does better, but when mages such as Hyped go fire, they incinerate (pun intended) the boss, like Omnotron with 71k dps, you'll never hit that as arcane. It also depends on your guilds strategies, if on magmaw your strat is to kill the parasites, fire can dominate on that fight (see: fire top record for magmaw vs arcane), on maloriak again fire will generally do better, although fire gets beat out by arcane on some fights, some of the time its the same mage holding two parses at once. It really depends on your playstyle, it wont be a changing factor in most cases but if you're REALLY good at fire, or REALLY good at arcane on some fight, theres no point in going the other spec.
That said, there really doesnt need to be another thread like this, use the search function, or have your brain function.