For a digital audio workstation, I think that you probably should use the maximum amount of memory that you can install.
I doubt that higher speed memory will make a difference in performance.
1600MHz memory will be easier to set up and get stable.
You should by 2 quad channel memory kits that are on the ASRock QVL list or buy kits that the memory manufacturer has tested to be compatible with your motherboard.
See if any of the DAW forums have specific recommendations for optimal memory speeds.
It's possible that some very intensive applications might benefit from slightly higher memory speeds.