Paladins will be tank healers, they will be sought after for this role, they fit this role well with their mana efficiency vs size of their single target heals coupled with beacon of light. On top of this the 'stereotype' that you get a holy paladin to heal will no doubt persist.
Disc priests also be tank healers. They will be sought after by those who understand their new utility but the stereotype of being a bubble spammer will hold them back in many situations where people don't trust a disc priest over a holy paladin, even if the two can both do the job fine.
A holy paladin will be able to be an all round healer. The instant casts, holy radiance, LoD as well as heavy hitting FoL enable them to heal the raid and anyone they want while keeping a tank up with beaconing and having alot of instant OMG buttons.
A disc priest however is fairly useless, as far as I can tell from beta logs at raid healing. They will be able to heal tanks and that's about it.
So where does this leave the two. I think holy paladins will be in more demand as they can provide a more all rounder to the raid composition while keeping tanks up; they are also seen as stereotypical tank healers.
Disc priests will be in a sought of void zone where they aren't quite trusted to keep tanks alive but aren't brought along for their general healing abilities (raid and tank). LEST WE FORGET THAT A TANK HEALER SHOULD BE ABLE TO HEAL 2 TANKS SUFFICIENTLY FOR 8 MINUTE FIGHTS, DISC UTILITY GETS A BIT SKETCHY WHEN YOU NEED TO HEAL 2 TANKS
In a 25 man it would be foolish to not have atleast 1 disc priest and 1 holy pally but if we are talking 10 man raids with usually 2 healers, most raid leaders are going to pick a raid healer and a holy pally as an hpally can keep two tanks up with beacon and has enormous raid healing capabilities as well. I play both classes and enjoy my pally more anyway so this is fine for me but food for thought for others without that option
To suggest that a holy priest, resto shammy or druid even comes close to single target healing utility in cata is ridiculous. EXCLUDING mana efficiency all healers can do some heavy heals (holy priest) but to believe that a holy priest could effectively heal a tank (usually 2) over an 8 minute fight better than a holy paladin or disc priest is preposterous