18% for 30 seconds + your arcane power and IV and other trinket CDs makes it worth more than a 3% dmg increase, if you use it with all your CDs its definitely worth more than 3%. This makes it better than lets say the DKs perma 3% increase.
The 2 set is absolutely incredible, I just got mine on my mage and I shot up a good 10-15% dmg just from that. Its essentially a 75% up time 13% haste, its so amazing.
Also, I would get chest first if its your biggest upgrade- my mage personally had the crafted 245 merlins chest piece. I had 232 tier helm. I upgraded it to a 264 sanct helm because it was a significant increase in haste spell power and crit + i LOST all that extra hit on the helm. This was a significant dps increase due to the gear i had available to me.
But lets just say every piece you have is 232. Get the chest > helm > shoulders > gloves. Get crafted leggings, BiS until you do heroic 25, its an incredible piece of gear, perfect stats for arcane.
I would not go FFB at all, its dps is significantly lower than either fire or arcane at any gear level. i would say stick with arcane until you can get the hit / crit for fire, then pick whichever you like. Fire isnt THAT much ahead of arcane, it is less movement sensitive though imo.