Firestorm is better?
Why is it better? Why is Astral Storm better? Damage?
Is Astral Storm better than Meteor Swarm? It depends. It depends on what Paragon Path and Epic Destiny you took. In short, it depends on all the factors that eventually end up on these powers when they're used. You can't say that a pizza is good until you actually eat it. Comparing pizza quality from pizza stores based on the size of their ovens is silly.
Why can Meteor Swarm be better than Astral Storm? Well, Meteor Swarm can be doubled up on or made into an encounter power using the Archmage destiny, and you can add damage to it from Bloodmage. Moreover, a Wizard has no second primary attribute, so he can raise Int as much as he likes with fewer repercussions. If you take Wis or Con or Dex or whatever, you'll be about the same as a Devoted Cleric seconding Cha, but your defenses are better. A Devoted Cleric doubling up on Wis and Cha is junking 2 attributes and none of his primaries deal with AC. He needs chainmail just to keep close to your AC, and he'll probably be inferior anyway. His Ref defense is going to be abysmal and there's just no helping that.
Well then, incidental attribute thingies aside, what CAN you do with Meteor Swarm? Let's twink it a little bit just to see how far it can go. A "normal" level 30 monster should have about 300 HP. We don't know because there aren't actually any standard level 30 monsters yet.
Let's burn an AP, use Bloodmage, and Bolstering Blood for 2d10. We'll also use Spell Recall to double up on Meteor Swarm, and Astral Fire to boost the damage by +3 more. We'll cede Spell Accuracy - one feat, to be able to use it easily, though that's not really necessary all the time. We can twink it more with Staff of Fiery Mastery, but that's a little hard to figure, so let's leave that out.
With just this, we deal 16d6+26+6+4d10+10 (ongoing). That's about 120 damage to the AoE effect - nearly enough on its own to bring every enemy within the target area to bloodied status. You can regain Meteor Swarm with Shape Magic, too. Or, you can make Meteor Swarm into an encounter power with Archspell and then regain it within the round with Soul Burn and blast the enemy twice using that combination. Then use Shape Magic to regain Soul Burn and do it again next encounter, then use Destructive Salutation and Meteor Swarm on the encounter after that.
I can imagine that using these factors, a Wizard can solo a level 30 encounter with Standards and minions all by himself.
Same with Firestorm. Closing Spell is comparable, and a Wizard can combo his Closing Spell with Evard's to layer on some serious AoE pressure, and he can choose his element type to get by resistances. A Wizard's AoE suite is unrivaled. Without using another daily power, he can layer Closing Spell, Combust (or whatever), Frostburn, and Winter's Wrath to get some serious AoE damage pressure going on, and then keep it on in between casting these spells with boosted Thunderwaves and Scorching Bursts
For a Devoted Cleric, we get Firestorm, Enthrall (or Thunderous Word), Mantle of Glory, and nothing (because none of the 7th level encounter powers for Clerics have AoE options).
Seriously, the Cleric's lagging here. Firestorm's good, but it's not good enough to make that up.