From my testing, esp. Sheen doesn't seem to act much differently than Fallof, although it automatizes the setup with simpler setup ("Roughness"). I didn't test it too extensively but doesn't seem like it reacts to light or anything of that sort.
The best velvet setup is still mystery to me :- ).
Apparently it really does work differently. Regular Falloff doesn't take light source directional vector into account but sheen does. Check out my previews :)
P.S. I figured out (partially) a relatively good velvet shader but still it's not the general solution that works in every scenario :D
I downloaded this into Photoshop but I am still not sure what I am looking at :- ) The light spread the sheen "specular" lobes? The lobes react do directionality/size of light?
I prepared more obvious sample that explains how it works :) The Sheen somehow reacts with count of the light sources and their direction creating this radial gradient instead of linear (when using regular falloff). I still got no clue if there is any groundbreaking advantage over well customized falloff but I'll do some more testing anyway. It may works with satin materials but velvet from my adventures with that particular material is (as said in previous post) more complex. Velvet must take into account few variables (third is most important):
1. Falloff/Sheen (done)
2. Specular with anisotropy (done)
3. Handling a 3D map/bitmap that allows to recognize what particular direction and angle the surface is to the light and camera. The real physical velvet piece (lets say a square 50x50 cm) looks different for every angle. It is caused by tiny hairs that leans always to a particular direction (in world coordinates) and once they lean towards You but if You rotate this piece of fabric they don't change their direction according to surface but they do so for the viewer/camera. Thats the basic principle that (from my knowledge) nobody handled yet :D