raid setup must be taken into account, if there are more then 1 warlock, lets say 2, then you can run 1 fire, 1 shadow. but if theres 1 warlock you haveta look at the other classes, if theres 1 shadowpriest, but a fire mage and an arcane mage then fire wins with Curse of Elements, as that will be 3 to benefit from CoE instead of just 2 from CoS. Fire is very viable once you get 950+ fire spell dmg.