I have not boxed the 4 priest setup past level 5, so take this with the salt it was made with.

I know there is at least one boxer on here that does the holy route, quite well as I understand it, but I would lean towards shadow for the following.

1) Mindflay - will auto direct you towards the target, helps with any facing issues
2) Renew is very powerful but can be wasted time/mana when you're just over healing your tank. With shadow dmg + VE at least you're always hurting the mob even if you're overhealing your tank
3) Being shadow doesn't mean you don't get renew, just reduces power significantly. Being holy means you don't get VE or VT
4) VT + BoW - lots o mana
5) Blackout x 4 = lots of mob stunning
6) Silence x 4 = 25 seconds+ of dead caster
7) Lots of gear with good + shadow dmg... not so much good + holy dmg
8) Also get Meditation from the Disc line for truly infinite mana but this means you get nothing out of the holy tree... meh

That's my take at least. And sure you could go with a non standard 50/50 build or something to try and get the best of both worlds, but my priest experience and Blizz's design fights that logic.

Alright you holy nukers, now for your side!