I would say ES even though your DMG is way lower but you will last longer.
Cuz you can reach 7k+ mana really ez.
But... Most battles don't really last that long. So you will blast away with your FB's and such and when they touch you, you can out live it, cuz without ES your most likely to die in one-three hits anyway.
But with ES you can take like 7 maybe and be able to get away and let your warmth re-heal your mana pool.
Even though some people will say BM!!! BM!!!!!
But you vs a 200 fcr fire sorc the other sorc is more likely to win.
So it can even out..