Fire is better than FFB, if you have the mana and the hit gear to do it. remember you will need to be at 14% hit (assuming spriest/boomkin in raid) instead of the 11% as FFB, or even the amazing 8% as Arcane.
The two are effectively dead even (fire and arcane) right now. It really comes down to whichever you like more and what you need it for. If you need really high burst (sarth3d for instance) arcane will be better. But for longer fights where it is slow and steady, they are about even.