I feel this is a matter of preference. My usual leveling talent path with a priest (and yes, I have several, gods help me >.<) usually goes something like this:
2/2 Healing Focus (Holy)
5/5 Spirit Tap (Shadow)
5/5 Wand Specialization (Disc)
5/5 Holy Specialization (Holy)
5/5 Divine Fury (Holy)
3/3 Improved Renew (Holy)
2/2 Searing Light (Holy)
2/2 Improved Power Word: Fortitude
(4 points of your choices in Disc - I usually took Martyrdom and 2 point of Silent Resolve or 4 points in Silent Resolve if I was grouping more than soloing)
1/1 Inner Focus (Disc)
3/3 Meditation (Disc)
5/5 Mental Agility (Disc)
1/1 Divine Spirit (Disc)
2/2 Improved Divine Spirit (Disc)
...and so forth.
That is my preference, and I found it worked better than Shadow for me and for a couple others I know (ask Vanea about his shadow experiences some time :>), also allowing you to heal with a bit more ease than you would have as full Shadow (although if you know what you're doing [and it's not hard, I promise

] and keep some healing gear, you can heal near anything short of Heroics and raid encounters as Shadow).
Mixing in a moderate amount of Spirit or "Owl" gear is very helpful no matter which path you take, because Spirit Tap = ah-may-zing (but especially so when paired with Meditation, imo).
If you decide to go Shadow, I would encourage you to spec Discipine until level 40, and then switch to full Shadow for Shadowform. Having played around with both a fair bit, I have always heard and found from personal experience that Disc is much more efficient (and better for staying alive) before 40th.