with this i would like to start a question / discussion of what type of diffuse shader is to use for which specific application. Cycles comes with a "Diffuse" Node, but other nodes do diffusion as well. Here is an example:
The Question is: What type of shader should be uses for which specific case? What are the differences? Should they be mixed?
Thank you!
Use diffuse for diffuse reflection, use glossy for glossy/specular reflection, and mix them because real materials also have multiple types of reflection at the same time.
It is a common misconception to think that diffuse reflection is caused by the surface irregularities/bumps, but this is wrong, perfectly smooth surfaces still can have diffuse reflection. Actually diffuse reflection is caused by subsurface scattering, but the scattering distance for most materials and rendering scenarios is smaller than the pixel size, therefore luckily we can use the diffuse node instead of the slow subsurface scattering node.
For more details, see my answer to a similar question: What is the technical difference between a diffuse shader and a rough glossy shader?
