i leveled smite and imo it went pretty well. the only issue i had was toward the end when the spirit nerf hit i had to sit and drink a little more. if you don't want to or don't have the money for dual spec this is what i would suggest if in endgame you are going to heal.
Im almost done lvling my priest (3 char) currently lvl 78.
I did shadow 10 - 25
Then i went disc, and really loved it, pulled everything and my mana was neverending.
Was disc from 25 - 73
then i decided to try shadow again, and it really went allot faster.
So now im finishing as disc / shadow dual spec. shadow for questing etc, disc for pvp / instances.
Ur spirit gear is fine for shadow no doubt, but play what u like
(personally hate holy, reminds me to much of ***"
sadly, you probably did this pre-Rapture nerf, when mana was litterally endless for disco
on topic: i used SoL/disc build myself in the 70s, it worked quite well, and don't be too worried about spirit gear
shadow benefits from spirit (a little bit) and has a built in 6% hit which is enough to get you going until you get some leveling gear form Northrend. Since spellpower was merged, there really is no pure healing gear anymore, it's all just caster gear (aside from gear with hit as healers have no use for it).
and as it was said before: spirit tap is king for leveling, regardless of spec, assuming you plan to do anything alone
I'd say stay holy for now, respec for spirit tap and go nuts, holy nova is amazing for the first couple of levels if you have good regen gear, you can pick up some +hit gear along the way and consider shadow spec when you get enough upgrades. I leveled from 70-80 quite fast as holy, but im in love with that spec so maybe thats why
Good luck in northrend.