I just want to say i am a lvl 70 holy priest and i have not had to solo often....but when i did it i was hard..
sure you can heal yourself but u usally pull more then you can handle.. when that happens you are basically screwed.
But i will also say that if you are holy you can find alot of help ... people always need someone to heal them.
Sure going thru instances gets you XP.. but will take you a while to lvl jsut the same...my advise is just create your toon to what you like...if you want to do damage then try shadow..
I for one like my holy priest it is easy to play and fun help others in groups...
I tried to go shadow at lvl 68 just so i could lvl soloing and to me it was just as hard. But i was not use to it so that may have been my issue...everyone is different on how they play .. so it all depends on what you like.

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*