Holy has tremendous throughput. Fully ToT geared I think a holy priest could solo heal ToeS lfr, maybe two heal. Holy shines when players are never topped off.
Disc is fun to play, you get to throw in some dps too. Disc shines on burst mechanics and when the raid can be topped off.
What I have learned is you can not go off of meters, they lie, they do not tell how well heal was done. A good feel for the fight and downing bosses is what is important.
When everyone is farming ToT and fullly geared Disc will be pulling even harder as everyone will be even more topped off. If you want to top meters go disc, if you want to have the ability to pull a sinking ship out of the water go Holy.
PS holy has bad mana issues compared to Disc. I also do not go holy for every fight, but you have to stand somewhere when it comes to picking up gear.