There are are currently two styles of paladin healing builds at endgame: one focusing on Flash of Light, and one focusing on Holy Light.
The Holy Light build is the pretty standard gem-everything-with-Intellect build that we all know and love. It relies on massive amounts of Intellect so that mana regeneration from Replenishment and Divine Plea is maximized.
The Flash of Light build focuses on Spellpower, and relies on the fact that FoL is very efficient to handle mana concerns. The idea is to get enough Haste so that your FoLs cast in 1.0 seconds (about 650 Haste rating), resulting in steady stream of very fast, moderately strong heals. It's sort of being like a living HoT. You generally need T9-level gear before this style becomes viable.
One thing to note is that neither build uses one spell exclusively. HL builds can use Flash of Light when damage is low to save mana, while FoL builds can still drop Holy Light when the going gets rough.
Holy Light's advantage is that it has huge raw throughput. Flash of Light's advantage is that it is "good enough" most of the time, is very steady healing, and you don't have to worry about using Divine Plea or the -50% healing Divine Plea entails.
My personal preference is Holy Light, because the Holy Light build is a bit more "forgiving" than FoL. Massive amounts of healing makes up for a lot of mistakes. If you use HL when you could have used FoL, oh well, you just wasted some mana. On the other hand, if you use FoL (even a strong FoL) when you should have used HL, someone might die.