Smite spec > Shadow.
(spec into shadow > Disc > Holy and after lvl 54 go back to disc)
At least in the early levels, Smite spec beats out shadow simply because although smite spec might kill slower it doesn't run OoM anywhere near as quickly.
Although I personally leveled as Smite spec all the way up to 75 where I switched to a disc PvPish build.
I did try shadow from 63-66 but I just couldn't get into it at all, I kept feeling like I was wasting mana on dots that only did half their damage to mobs before said mobs died leaving me with a mana pool that dried quicker than a puddle of water in the desert.