For solo leveling nothing beats shadow, you can basicly just spam dots on as many enemies as possible loot and have 100% mana/health
for dungeons I found the only difference between holy/Disc until 82ish being that with disc spamming 1 button (smite) and holy spamming one button (Heal) I did semi-competent damage while being disc.
I have been leveling a priest recently. Currently on lvl 62. Shadow is the way to go when soloing, and you should pick disc as second specc on level 30. Go for a smite specc in disc, its awesome for dungeons, and fun also.
You can check my char: Minitor on Deathwing. I'm not saying I specc 100% accurate, or claim to be pro at priest, because I aint, but still, I think I've specced well enough