Usually decides on the decorations.
Because I have seen written instructions given on how to decorate the various pieces. Although this could be decided very early in prototype stage, and the "bean counters" or marketing people might make the final decision in choosing the "look" for a watch at a given level in the brand (and how it fits in the larger group, like SWATCH).
Or sometimes even a specific influential customer... "advises" on the look for his piece
Cazalea