If the picture in this link is any example, extreme though it may be http://www.mh-audio.nl/DiffusorCalculator.asp then diffused reflection is best.
This studio mixing room is at Blackbird studios and was designed by legendary  engineer George Massenburg. The extreme depth of the prime number 2D skyline type diffusers that cover the entire right and left walls as well as the center are probably solid hard wood sticks of various prime lengths and sequences allow for coverage in the way down in the bass and diffusion/scattering waaay up in the audible rage. Calculating such a diffuor and assembling it must be a monumental task. I do not think the full coverage is a repeating pattern of square cells made up of severl hundred sticks per pattern but rather an extremely huge prime number sequence to create enough cells (sticks) to make one large diffusor. Cant be certain about that but I also doubt there is any absorption stuffed in the wells of those monstrosities. It is is just so damned good at diffusion that the expanded diffuse ambiance of the room should create the sense of no walls at all yet subtle spacial cues that are only good for the imaging and sound stage. Notice in the corners the absorption and probably bass trapping. Also smaller skyline diffusion hanging from the very tall ceilings. I could just sit in that room in silence and feel the bliss.