Back in the days when I use to race, I had this conversation once. Most people would say ball of the foot will use more muscle fibers to generate the same movement, generating less fatigue through time. Others will say that more fibers being use will consume more O2 and generate more fatigue through time. After about 2 months conducting timed interval training and heart rate monitor analysis (same bike, same diet, same circuit, just different pedal position) ball of the foot was more efficient for me. It might not be an universal rule though: my bike geometry, muscle conditioning and overall psychological effect of one style may not be the same for eveybody. I suggest a semi-scientifc, controled test for about 100 days.