Diffusers vs Room Lens

Is it better to have one or the other, or to have them both?
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).
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.