I leveled as disc even thought I was disc/shadow speeced from 70-80.
The reason was that killing stuff was faster as disc and you had a lot better survivability - Just holy fire, smite and maybe a second smite, instead of starting to dot up.
So I'll say go with Disc for leveling, I belive it's more versatile than holy when you have to quest and such.
I think my spec was something along the lines of this ->
http://www.wowhead.com/?talent#bVcbu...xt00bZh:ubVcz0 get all the non-disipline talents first and and then add in discipline as you go along.
Again this is only useful if you solo stuff.