It won't make much difference just for levelling.

I personally do (did) prefer demonology and levelled both of my warlocks that way, but I haven't actually played a lock since level 70 so I'll bow to other peoples' more current knowledge on that one.

Because you're entirely focused on getting a whole range of guys to 80, rather than optimising a group for instances or PvP, you can pretty much get away with anything.

I once played my hunter team for a week before I even realised they'd had a talent reset......