Inner Will or Inner Fire? (Question for Jhazrun)

[Thotz]

I was wondering that is there any certain moments to use Inner Will instead of Inner Fire a way that it will be more benefical in Firelands 25 heroic as Discipline? Is the 15% mana cost worthy while using a lot of PW:Shields and is PW:Shield spam better than Prayer of Healing spam to all 4-5 groups?
I was looking at your raid's logs today and I saw you almost using only PW:Shield, for example on Bethi'lac. Also you were using most of the time Inner Will as Holy in Ragnaros 25 heroic fight, which made me very curious. I have always thought that Inner Fire is the one-and-only for both specs. Could you please share your thoughts about what Inner-spell to use in different situations? =) Thanks!

[Jhazrun]

I could happily finish a raid without leaving Will once, but the fact that the recent boss nerfs and our gear mean I can't cast ProH without overhealing half of it is probably contributes. Fire is still useful if you're actually taking enough damage to chain those during Bloodlust.