For a priest, I far prefer Discipline to Holy, as far as healing goes.

Discipline is more about preventative damage then the big heals once someone is hurt.
It does however have better twitch healing (Penance) against unexpected damage.
The mana efficiency is pretty insane.

As far as keeping a single target up, no other healer spec is as good.
Holy has better AoE/Raid Healing, with their extra talented heal.
But Disc is not a slouch for AoE heals.

For PvP, I usually go deep Disc, with 2 points in Holy for 70% less heal interrupts and then Shadow to Mind Flay.
But for PvE, its usually 58/13/0 or similar.