I'm sorry if this has already been asked.

Hi guys, I just reached level 90 on my priest and i want to do arenas as a healer.
Ive always seen disc being the dominating spec for pvp as a priest, but lately ive seen so many on streams playing holy aswell...
my question:
Whats the best spec to reach high rating? (in setups like: RMP and MP in 2s).
Whats the pros and cons for both specs?
thank you for youre time