I have a lvl 13 priest on durotan, don't know too much but if there is anything in that general lvl, let me know!

I have an undead priest currently lvl 11 on the Whisperwind realm, don't really know much about them yet but I'm specing shadow, I hear thats the way to go.

Yeah, I believe shadow is the way to go for basically 1-70, or at least to 60. You could go holy or holy/disc for leveling but you would have to be strictly a instance king/queen.

Best idea would probably be to level shadow, and keep healing gear on you for instances, should be more than enough power to heal anything pre 70.