It depends really on what exactly you want to do other than just heal your party.
And also on how your play style develops.
Pvp wise, if you go as a healer/ utility spec, a priest allows you to be mroe aggressive than a pally but a pally can be a much sturdier rock for taking hits than a priest.
Pve wise, again it depends on your play style, pallies are great healers, too mny go by the old adage that they spam flash of light, any that simply spam (this goes for any healer) should be shot.
pallies have a lot of great tools depending on the situation as do priests. Priests have the ability to actively mitigate damage via shields and so forth and more so through talents depending on if you go disc or holy.
Pallies healing wise are mroe of the focused main healer type due to teir low manacosts on spells when compared to other classes.
Also shamans are great healers themselves, and to those who say dont roll a shaman as they are nerfed, throw their opinion out of the window, the majority of them just miss the fact that they cant get away with spamming chain heal as they did in late tbc content.
Shamans are still great healers, for pvp , parties and raids.
Personalyl i am now levellign a horde priest to play with mates from work, but thats due more than anything to a personal preference, since i find priests the most fun class to play.
I love my Shammy for his amusing WF crits, but its just irritating when if you spec Resto or Elemental, simply put, you're gimped. Hell, they even make Enh's totems overlap with DK's Horn, and Pallys buffs. What does a shammy bring to the raid anymore?
Most classes have had their buffs and utility given to other classes in the hopes of a "bring the player not the class" attitude.
it works, yes shamans might not be the strongest healers but that means they're far from gimped, chain heal while not as strong as some spells is as strong as it needs to be really, buffing it would make it overpowered, as it was in tbc.
the only thing lacking is a spell that has a lwo cast time with a low to meedium throughput, or an additional hot-based ability in order for them to be able to actively heal better.
