I have a level 70 shadow priest, when discussing whether or not to go holy, you need to think about where do you see yourself in the future. Do you want to be a raider, or someone who gets full vengeful in pvp. I find that holy priests and shadow priests are good at pvp, i chose shadow because its preference, either one is fine. Both are actually good at raiding as well, you will be asked to heal as a holy priest, but resto druids/ resto shammys will be called as well. I am a 70 with a mix of gladiator and raid gear, i love shadow, such an array of talents to choose from. Just like anything, some raids and premades wants holy priests to heal, or some raids want shadow priests to dps. Its all a matter of preference, leveling is the same, it doesnt matter, choose what you want and thats it, dont apologize to anyone when you say, "sorry i am shadow".

*Just dont go full discipline, either hybrid talent or choose one or the other*