From a design & marketing standpoint ... the perception of quality outweighs the actuality of it. I juggled the Riv vs Mercian question as well and chose Mercian based off of the perception of a higher quality. Quality being defined by the reputation and history of the company, the optional features (tons more than Riv and for the same general price), the color selection, and the fact that they actually make the frame. A guy named Derek built my frame.
Weighing all those factors (which were important to me and might not be to others) ... it was clear to me that Riv was 'less bike' for the same money. They charge you an additional $350 bucks to alter the color (if they wave their magic wand of approval) ... come on!