Fireball damage: 40 single cast/96 dual cast
Firestorm damage: 100
1/2 Augmented Flames
Fireball: 50/120
Firestorm: 125
2/2 Augmented Flames
Fireball: 60/144
Firestorm:150
2/2 Augmented Flames and Aspect of Terror
Fireball: 70/168
Firestorm: 160
Conclusion:
Fireball is a far superior spell, boasting almost the exact same damage when dual cast--with a negligible difference or even being stronger with 2/2 Augmented Flames and Aspect of Terror--while costing significantly less, having a much shorter casting time, allowing for mobility, triggering Impact, not getting weaker with distance (allowing you to deal full damage without hugging an enemy), all while maintaining a smaller (but still respectable, IMO) area-of-effect of 15 feet. The smaller (but still useful) AOE, in fact, may be seen as a good thing: you can use the spell without hitting allies.
The difference in damage between the two spells got higher as their respective power increased by the same percentage, the difference being 4 points at a 0% increase in power, 5 points at a 25% increase in power, and 6 points at a 50% increase in power. Aspect of Terror adds a number--not a percentage--to all fire damage (+10 points), and 10/60 is a bigger fraction than 10/150, which explains why Fireball benefits more from that buff: it gets a higher increase in percentage than Firestorm (16.6% versus 6.6%). This means that Fortify Destruction potions will further pronounce the difference in power between the two spells, where Firestorm will always be stronger.
Still, unless we're always going to use potions to cast spells, I don't think that a maximum 6 point difference in power is enough to justify immobility, long casting time, enormous casting cost, and a damage drop-off at a distance. For that matter, 150 points of damage is not nearly enough to justify being defenseless and close to your enemies for as long as it takes to actually cast the spell.
As long as they have the same increase in damage (+0%, +25% or +50%), the other two elements deal more damage than Firestorm with their master spells, with Lightning Storm outdamaging it in less than two seconds and Blizzard dealing more total damage (a minimum of 200 over ten seconds, which is higher than Firestorm's maximum). Isn't fire supposed to be the most damaging element?
What gives?