LCD:
The answer depends on the size and reverb time of your room[s].
To really get your arms around the concept/question you need to grasp the concept of the Schroeder frequency.
There is a cut off frequency below which modal behavior dominates and only absorption can be effective as an acoustic treatment. Above this cut off, diffusion becomes a practical solution. This range varys with room size [larger rooms can use diffusion effectively on larger waves].
To calculate this frequency you divide the room RT60 by the volume in cubic feet, take the sqaure root of the result, and multiply by 11250.
Using a full featured room mode calculator will calculate this cutoff for you for a given target RT60, and help you choose which type of treatments to use.
Here is a link to a spiffy web based room calculator you can use for this purpose.
http://www.bobgolds.com/Mode/RoomModes.htm
Here is a downloadable excel based room mode tool that will calculate the schroeder frequency for you - and just plain rocks [click on "Room Modes and Reverberation Time Calculator" to download - I think you have to log in - registration is free and easy]:
http://forum.studiotips.com/viewtopic.php?t=1414
As to why use diffusion... here is an informative explanation:
http://www.acoustics.salford.ac.uk/a...sign/index.htm
And this book contans a good but easy to understand overview of the topic:
http://www.amazon.com/exec/obidos/AS...486553-8829465
As to material or cost considerations, obviously the laws of physics don't give a hoot what you pay for the substrates in questions. A "cheap" plastic diffuser properly configured will work just as good as an expensive wooden one.
A very inexpensive and effective design is to simply bend a sheet of thin plywood into an elipsoidal curve [FKA polycylindricals, or "ploys"]. You can makes these very eaisly or buy them with nice built -in hanging fixtures, etcetera.
Scroll down to read about polys:
http://forum.studiotips.com/viewtopic.php?t=48
or look at these babies!
http://www.rpgdiffusors.com/products...dial/index.htm