I suppose with ryzen being so new it might pay to get a ram kit that's certified to work with the chip/mobo.
But that aside I'd just get whatever is cheaper at 2666 or higher.. I'm guessing you can always down clock it if they don't like the higher speeds. Maybe wait for some real world bench marks, it doesn't make much odds with Intel hardware, ryzen may be different.. Who knows until some real world testing is published.