Left for front as aside from most bikes coming from the factory that way, It really makes sense with calipers with their cable anchorage on the right side. Having the lever on the opposite side made it more possible to have a very smooth arc on the brake cable and housing from the (non aero) brake lever to the caliper, resulting in lighter and smoother brake action..... I also noticed that crossing over with the brake cable from one side of the caliper to the other seems to help keep the caliper centered longer.