It depends on the room. Each have different acoustical properties. Some rooms will benefit from one or the other or both. However, almost all rooms need some level of mid-band diffusion (there are exceptions of course).
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I have a 28' by 18' by 14.5' room. I used to have six 2' by 8' columns of RPG copies which were 16" deep. Additionally along the side I had 4' by 4' raised 6" deep RPG copies. All of this diffusing was substantial better than the Tube Traps that I had before, or the big panels of sound absorbent foam I had earlier. I bought 3 Room Lens. They did help focus the sound somewhat. Finally I took out the RPG panels and wow, everything was more coherent and focused. Later I added 12 more RoomLens. Where you put them greatly shapes the sound.