Both disc and holy will surely be fine at 85. Blizzard won't abandon one specc, no matter what you might think.
The real question is, which one will be more fun. I hope disc, just cos i always liked disc, but holy is getting the new chakra system, and the most awesome disc talent (archangel)is not a healing talent...
Unless disc healers are supposed to heal by damage, but i don't think so. If i understood it right, archangel and such are only there to give you something to do if you overgear the content or solo (or in PvP). So basically, most of the starting disc talents are completely useless (at least until you start outgearing content, and that won't happen for some time).