I'm surprised no one has mentioned Moerch arms which are probably the best candidates for testing 9" vs. 12". You can easily swap a Red 9" wand for a Red 12" wand, and the mass will be the same. The only difference is the length. Something obviously must be different if they have the same mass. Thinner wall thickness perhaps or less damping, I don't know. However, they are certainly made with the same design and materials as well as wiring and mechanical connections. Unfortunately, I have never made the comparison myself. All of my experience with the Moerch has been with the 12" Red wand, which sounds very good but I prefer the 9" Triplanar VII. That proves nothing of course.
Dave
Dave