If raid healing is all you're concerned about holy is the better spec for this job at the moment the disparity will probably become even bigger after the recent divine hymn buff but discipline is still very much needed for the mitigation it provides aka barrier,shields,pain suppression and the brand new spirit shell.
With all that been said though both specs have something in common at least and that is terrible mana regen and mana costs, as it is at the moment if you don't spec solace the chance to last a ~10min fight mana wise is very low unless you plan on afking half the time, hopefully that will get addressed soon.