True atm Disc > Shaman quite sure but when Locks get stronger (as they will), that's a shaman buff aswell as they can deal best with locks due to Grounding/Shock/Tremor, much better than a disco.
If next season gets dominated by Lock teams than shaman will be the clear choice, if not Disc will most likely be better.
But at this point in time we will have to wait for further PTR patches, testing and see what happens next season.
I don't think anyone can tell with 100% accuracy what healer to go with atm.
I'm sorry if this answer doesn't quite tell you what you're looking for, but it's the best I can come up with atm