I would like to try fire, give it another chance. Specialy since 4.3 and all the buffs. In the past, I had a arcane spec and a frost spec. Advantage was that I didnt have to completely reforge my gear every time since they had more or less the same stat priorities. ( or so I read in the guides ).
Now if i go arcane / fire, I would have to go back to town and reforge every time when i switch specs. Since I only do heroics and maybe a occasional LFR raid, I doubt that the random people will wait for my reforging after every boss...
So assuming that you only have one gear setup and need to use it for the entire instance, what is best ? Stay arcane, play arcane, and keep my frost pvp spec ? Or go fire, play fire, and ignore the fact that i will have fire stats with my arcane spec if i switch for one encounter ? Or switch to fire pve and frost pvp spec ?
Personaly i have no clue. I am leaning towards fire pve / frost pvp.
Only the Sine of DW fight sems to favor arcane over fire, the remaining fire is equal or better than arcane from my understanding of the situation. If you want to play fire go ahed if not stick whit arcane, it is wry close inn the average fight, and better where burst is needed.
Frost requires different reforge than arcane. If you're not crit capped for shatters you're doing it wrong.
As for which spec, you can just play fire and do acceptably without needing to reforge. Arcane still has better consistency and properly controllable burst, where fire should technically end up doing a little more damage after RNG averages out.
Fire and Arcane are close enough this tier that you can pretty much just pick the one you want, gear for that and stick with it for every boss fight.
There are no fights like Alysrazor where one is a clear winner, while some (like Spine and Ultraxion) favour arcane and others (Yor'Shaj and Hagara) favour fire, you can still perform to almost the same extent playing as the other spec.
I wish I could say the same for Frost as it has the rotation I find most interesting but I'd advise against playing Frost. Unlike Fire it provides the same niche as Arcane with less utility and less damage, not really worth it.
You're going to have a better time and perform better with fire in 4.3, overall. We've got fights with movement and multiple targets now, and even on a patchwerk fight fire outperforms arcane now according to simcraft and rawr. Keep an arcane spec for the 1 or 2 fights that it favors.