As I see it, the question is one of playstyle and reaction to what the other team is doing:
-You get focused, you need to put up Fire
-You need to kite, you put up Will.
-You push a lot more high-mana spells and go OoM faster, and you need to throw up Will anytime you're not getting focused, or suspect a switch is coming.
-You feel like you aren't getting enough throughput, you need to keep up Fire.
-You feel as though you don't have enough throughput and you OoM too fast, you need to reevaluate your playstyle or your teammates.