They really buffed a priests holy damage towards the end of BC, so solo holy and disci levelling isn't the impossibility / exercise in masochism it once was, but shadow is the way to go for dps / solo play. And if you keep a set of mp5 gear in your bags, you'll also heal quite adequately for 5 man instances right up until lvl 80 HC's, though holy or discipline would be the preferred and easier way to do it.
So far as choosing between holy & discipline goes, I can't really help you there as I've only ever played holy on mine. From what I gather disci seems to be more focussed on group damage prevention using multiple and stronger shields than holy, while holy focusses on bigger and faster heals and is more suited to single target healing.
I know I've enjoyed holy on mine, so I'm quite happy to recommend that to you, but it might be an idea once you get to 80 to dual specc disci & holy rather than shadow / ????, and swap between the two while healing instances at HC / end-game level to see which you prefer.