Final Conclusion, without (possibly broken) enchanting:
One Handed: 680 damage for 10 seconds, followed by 211 damage, with 680 power attack damage, average 367. With stamina potions stabilizes at 680.
Destruction: 146.25 damage per second for roughly six seconds, followed by ~28 damage per second indefinately. With magicka potions, stabilizes at 146.25 damage per second.
I just wanted to throw my two cents into the pile. For reference, I am a level 52 pure mage (0 combat perks outside of magical skill trees). I rely on Destro for ALL my damage. I have 0 perks in conjuration and have only learned the soul stealing spell from that school. I have many alteration spells, many restoration spells, and a couple of illusion spells I almost never use. I also have 100 Enchanting and Smithing, but ignore alchemy (I never remember to use the pots). I play on Normal difficulty.
Anyway, my two cents. You make a very valid and indisputable point that dual wielding weapons is WAY stronger than dual wielding spells. Now I'll make my point...Who cares? 146.25 damage per second is WAY more than enough to floor the entire game, dragons and dragon priests and super druegar and everything. I know, I've done it (and at a lesser amount of damage than 146.25, since I never use alchemy). So if anything, dual wielding melee weapons needs a MASSIVE nerf. Saying Destro needs to be as broken and dual wielding melee is just silly.
As for difficulties above Normal, I have no idea. However, I do know that all difficulties above normal are supposed to be HARD. Possibly SUPER hard. Maybe Super-Insane-Rediculous-You-Get-The-Idea Hard. So while Destro may be much more challenging on harder difficulties...isn't that what you're going for if you play on those harder difficulty settings? I mean, which is more important to you when you play on a difficulty setting higher than normal; having a challenging game play experience or finding the silly broken combo that trivializes what should otherwise be a challenging game play experience? If it is the latter, than sure, Destro needs to be "fixed." But if you play on harder difficulty settings for the latter reason...uh...yeah...get a life? I am not really sure what to say to you. LOL
If you play on harder difficulty settings because you want a more challenging game play experience...why are you complaining it is more challenging?
Anyway, that is all...