Well Fire is suppose to be more damage focused than Air.
But it's not supposed to be focused on creature generation.
The questions are "How hard is it to remove Malebolgia?", "How much of a cost increase would this be worth?" and finally "Where should the cost increases be implemented?"
It's reasonably difficult to remove at the beginning, requiring Shockwave, Rage Potion, Lightning, or boosted bolts, and it only gets harder after that, where I first have to hit through other creatures before I can even begin to damage the Malebolgia.
Where the cost should be implemented and how much it should cost are basically decided by which direction you want it's vulnerability to be. Do you want it to potentially be a big useless drain of quanta that's easier to get the wall going for, or do you want it to be harder to get the wall going but potentially have a lower loss?