I too am coming back to wow very soon. For those that know me, you were right, only took a couple weeks of FFXI to get sick of it again.
Anyway, for the point of the thread, I agree, affliction is best until at least 56. Affliction is often regarded as the best levelling spec, though Demo is still very viable if you do not try to use the Felguard until 56, he has a few other talents he needs to make him worthwhile, namely Imp Health Funnel, 2/3 Mana Feed(3/3 if you like, but 1/3 is not enough), and Demonic Resilience. Without those, he just doesn't seem to work as well.
That said, I tried it out from 54-60 and ended up going back to Affl. It just suited my playstyle more. You may feel differently though.