I guess it really depends on your play style and how much you want to do solo. I leveled as disc/holy and it took about 11 days /played, but that was with a fair amount of time spent maxing out professions and just exploring. So while shadow may be a bit quicker, I don't think it's significant, unless you insist on doing everything alone. The benefit I see in leveling disc is that it pushes you to work with people more and to get comfortable with the healing play style. It seems silly to me to spend 59 levels using one set of spells, then completely switching over at 60 and having to unlearn the habits developed in all the previous levels. Plus focusing on healing from the beginning will help in befriending a number of tanks, who will end up being there for you whenever you want to run dungeons. If you start off in a friendly, helpful guild, you're going to level quickly with whichever route you choose.