They are two distinct playstyles.
Holy is throwing renews and ProM and waiting to throw that monster gheal till it's needed. Guardian spirit is an oh sh!t button that will stop someone from dying and they have a quick group heal that provides nice utility. Mana is conserved by allowing regen to tick between throwing big heals
Example Build: http://www.wowhead.com/?talent=bVcbzhZfxtcs0qihVcst
Discipline is constant barrage of heals meant to keep a tank topped off. Power Word: Shield, renew, ProM and once the tank gets 7-9k down penance. Flash heals while Penance is on CD. Gheal is an emergency heal for a tank down 9k despite your efforts while penance is on CD. Pain Supression is an oh sh!t button that reduces damage taken to allow you to play catch-up. Mana is conserved via crit healing creating mini-shields and rapture returns mana based off of direct healing spells.
Example Build: http://www.wowhead.com/?talent=bVcbuhxtbxoicRtbxtc
Both work very well at what they do, Holy is great for group healing and utility (good MT healers) Disc is a top-notch tank healer, who, like a paladin, may suffer in group-healing situations, but can easily pull through with work and is a very active playstyle.