But which is "better"? *drum roll* It depends!
Benefits of Leveling as a Healer:
-You have an easier time getting groups
-You have the time to become familiar with your healing tools in bits and peices, rather than having them thrown at you all at once at a higher level
-Getting healing gear is easier (presumably, as the healer in the group, you have little/no competition when it drops)
-Practice makes perfect
I will sadly never be able to recreate the healing experience I had with Ambrosine. She healed 5 man and 20/40 man raids at 60. She healed leveling to 70 and did some more 10 and 25 man healing there. Then she healed her way to 80 and thrashed and flailed her way through 5 mans to 10 and 25 man content. I have healed a looooooooooooooooooooooooooooooooot on that toon, in all sorts of circumstances. I had greater finesse with her at 80 than I will on my priest for a long time.
Cons of Leveling Healing:
-Soloing can suck (unless you're a disc priest. Weee holy fire!)
-If you're on a PvP server, I have one word for you: bait. Lyr can defend herself to a point, but Ambrosyne is well nigh helpless. C'mere and let me holy shock you to death!....once every six seconds!
-Soloing can suck A LOT
Pros of Leveling DPS:
-You kill things/level faster
-You can defend yourself in PvP
-Big numbers are fun!
Cons of Leveling DPS:
-It can be harder to find groups
-You get less time to hone your skills
-It can be harder to get gear for later healing (less so if you run with other kinds of healers, but you still might fight over rings and cloaks)
-You might get more irritated with trying to do things as healy specced later (dailies as heals = D: )
Yes, the list of 'cons' I have is longer, but the whole 'killing things faster' is pretty heavily weighted by most people.
If you chose to level as DPS and respec heals, there's a couple of ways to handle it:
-Respec before hitting max level. DPS to 70 and then switch and heal your way through Northrend
-Respec at 80, but throw yourself at a lot of normal instances until you get the hang of things. Reading strats just isn't the same. Progress to heroics later, when you're certain you have the gear for them. Being undergeared in heroics just makes for frustration. Even if you happen to have the gear for heroics right off the bat, they're not the place to start learning!
Lyr learned in the Nexus that if your level 72 Death Knight tank in blues and greens makes pulls as if he were on his 80 warrior in blues and epics, you die!