Because I'm too lazy to read through 4 pages of things
I'm going to just go ahead and post what a mentor and I though up of as the reason for the two robots. We figured that since the fricitional force is the normal force times the coefficient of friction, then a heavier machine would generate more traction. But according to Newton's First Law, an object in motion wants to stay in motion, and an object at rest wants to stay at rest. Therefore, for the heavier machine, the inertia of it is far larger than that of the lighter machine. We both felt that the four motor machine was heavier, but could not produce significantly more traction to counteract it's inertia. Therefore the 2 motor machine accelerated faster only do to its smaller inertia.