I know there is a priest leveling guide right now but the Smite Spec is the best option before level 40 due to the fact it can produce more damage at lower levels, mind flay really does nothing until you get shadow form, and other such talents at the bottom of the shadow talent tree.
Here is the Smite Spec I recommend pre level 40, blessed recovery is useless, don't pick it up you will be shielded a majority of the time and the likeliness of you being crit when your shield is down is highly unlikely and blessed recovery hardly heals for anything.
Here is the Shadow Spec I recommend for level 40+
Smite Spec = Glyph of Smite, Glyph of Inner Fire
Shadow Spec = Glyph of Mind Flay, Glyph of Shadow Word Pain, Glyph of Dispersion