Destro: Great dmg, but has mana issues until higher levels
Demonology: I find kinda boring
Affliction: Good dmg, never has mana problems, has loads of selfhealing which is good for keeping yourself up so theres no down time, + once you get all your dots on a mob its basacly dead...this is realy good if you get soulswap+glyph as you can dot somthing up then just run around putting your dots on everything
Stay Affliction until 60ish, After that all 3 look good and you can choose based on what you enjoy most.
I've walked the realms of the dead. I have seen the infinite dark. Nothing you say. Or do. Could possibly frighten me.
We are not monsters! We are not the mindless wretches of a ghoul army! NO! We are a force even more terrifying! We are the chill in a coward's spine! We are the instruments of an unyielding ire! WE ARE THE FORSAKEN!
Those who do not stand with the Forsaken stand against them. And those who stand against the Forsaken will not stand long.
I'm of course an affliction-fanboy since vanilla, so take my opinion as biased, but Affliction is pulling ahead so easily when levelling, in my experience. Both from 10, from 60, from 80 or wherever you were when the patch hit.
It might be that it's the specc I'm most intertwined with, but it's also the specc I'm most entertained with.
Since you now have UA at 10, you should throw all your DoTs up on one, while sending pet on a second, and then DoT that one up as soon as you can (to tag it, at least) and stop as little as possible.
Once you have Curse of Exhaustion, UA->CoX is a very good way to ensure that mobs never reach you, before dotting up the rest.
Then just keep pulling as usual.
With that said, the two other options are entirely and easily viable. It all comes down to what you enjoy.