Cool to hear your rolling a healer. There just doesn't seem to be enough of them around.
I have a Prot Pally and a Shadow Priest (that has bounced between holy and shadow about six times now). I've also speced out my pally to holy ONCE, and came back fast to Prot.
My personal opinion is that if you want to heal be a priest. The funny thing is that I really didn't see much difference between the two (and Holy Pally's can't tank, well they can, but so can mages lol). ALTHOUGH... I guess this is one way to say it... You can use a butter knife instead of a flat head screwdriver to get the job done, but it's better to use the screwdriver.
A. Don't forget that both classes can de-buff, though I think the Priest has the upperhand on that... that being said.
The crits on a holy priest are really phat, the total hpm is better with a holy priest and they heal really well for groups. So when you run an instance you can keep your tank well healed, and the rest of the party above 50% too. Everyone loves the extra Stam with PwF and shield bubbles are a saving grace. Something to think about is that if you build a Holy Priest... that's what they do. Dps is not in the books. To lvl up it's way easier to run shadow till at least lvl 60. At 60 you can run instance like crazy... because everyone loves a healer!!! But most of what I've read says Shadow till lvl 70, then convert. My priest is lvl 50... and I have to say it's going to be hard to convert over... I like playing a shadow priest, and my plan is to go holy at lvl 70.
I've had a Holy Pally with me in a BRD raid while I was tanking. I loved this guy. The extra aura and blessing was cool. And though he really didn't do much for the others, mostly cause I was the tank... and done right a Prot Pally ROCKS as a tank, and the rest of the group DPS'd like mad, killing them FAST, it was a good group... but he did through in a few heals around the board.... a fun couple of runs all and all.... anyway the Holy Pally rocked at keeping me alive. As a tank healer, (a one man focus) I really believe that nothing compares with a Holy Pally as your right hand man. Blessings, Aura, AND healing!?!?!?!?! Where does it go wrong? Throw in the greatness of bubbles and some of that instant holy damage and you have a cool healer. Again, lvling is prot or ret till 60. But at 60 you can go holy and never look back.
So it's really a question of what kind of healing you looking for?
Groups, Raids, and party heals a Holy Priest can keep up with it.
Tank Heals (and support) and a bit of off healing Holy Pally.
Again, just my opinion, and I am still new at all of this. These are my first two toons... I just felt compelled to put in my 2 cents.