2 Thunder/Lightning Storms deal 4 damage and costs 6/4.
1 Rain of Fire/Fire Storm deals 3 damage and costs 7/5.
You missed one thing. The cost of 2 Thunder Storms is 6
+two cards. The quanta cost of Rain of Fire is higher, despite dealing less damage, but it is only one card. If that second card was a Fire Pillar, you would easily make up the quanta difference.
decks also work differently to
because a lot of Fire cards gain from having a lot of Quanta. Air keeps Quanta supplies low, and repeats a lot of abilities like Dive and Eagle's Eye which cost
.
Fire can also surprise an opponent by using Immolation to get the Quanta when they thought they were safe. In unupped play, an advanced player will be very sure if his opponent can afford Thunder Storm or not.
Despite all this, I still don't think it would be the end of the world if unupped RoF cost 6
. I think the upped cost is correct.