The only difference is that there is no difference besides the finish, it is the same material, but with "Carbon" the outside layer is painted. Oh and accessories, instead of two cables and one set of ear pads you get one cable and two sets of cable.
If you have original NH, there is literally no reason in spending money on Carbon. In terms of sound both are the same.
If you have original NH, there is literally no reason in spending money on Carbon. In terms of sound both are the same.