I'd pretty much agree with Alcattle.
Affliction is by far the most mana efficient, especially when leveling.
Which means the least down time.
Although, with 5 toons each only expends roughly 20% of their one-boxed resources per pull.
So it probably won't make a huge difference any spec.

If you have a spec in mind, for once the team is 80th across the board.
I'd probably go with that one.
Just so you can set up your hotkey for Shadowfury or instant cast Howl of Terror, and learn it as you go.

If you don't have a spec in mind.
Affliction till 50th.
Felguard at 50th, eventually building towards Ruin in Destruction.
Ending at 0/50/21, at 80th unless you have a different spec in mind.