I use Inner Fire on every fight except Artremedes. For that ~1 week period where bubble spam was viable, I used Inner Will. But before that, and after that I have used Inner Fire and have never ran in to mana problems. In fact, on most fights, I still have quite a bit of mana left so I start carelessly shielding everyone near the end until the fight is over.
It's perfectly fine to stay in either, or to stance dance between the two. A nice speed boost never hurt anyone. More spell power is also nice. I wouldn't rely on someone telling you what to do. I'd play around a little and find what works for you.