I am a firm believer that it is not what spec you are playing but how you play the spec you have that makes all the difference in the world.
Gear and spec can make things easier or harder but in the end it is the person behind the screen and what kind of player they are that makes the difference.
My horode 70 is a Holy Priest who's guild is in the finaly stages of getting Kharazan on famr status so have had a bit of experience in the past few weeks in healing 10 man groups. My guild always brings three healers into their Kharazan raid so there is plenty of healing to go around.
The absolute best tank I have healed for so far was a very well played bear (Druid). Not just because he had tons of health and armor but because he was VERY good at grabbing back agro from the erant monsters that took a liking to another party member (usually a delicate caster type).
So I would say play what you want and just find out all yuo can about your class and spec choices to be able to make the most of them in a raid situation.