When lvling my own priest I did Shadow up to around lvl 64 then I switched to Disc. I would say go shadow until you get to Outlands then go Disc. There really aren't bad problems with multiple mobs until Outlands anyway and you can still heal instances as a Shadow priest in early game instances. Being Disc will help a lot in Outlands to keep yourself going while you fight several mobs. That is also when you get penance which is the best part of being Disc. And as Disc you can very easily heal most 5 man instances with a tank who can hold threat. Disc is best for 5 man groups and for tank healing but Holy is a much better raid healer.