I have both a mountain bike and a road bike. Looking around, I have noticed that the mountain bikes use the 2-hole SPD style pedal while the road bikes typically have the longer 3-hole style. Why is that? Is there an advantage to using the 3-hole type on the road bike as opposed to the 2-hole type? I've heard that the larger platform of the road pedals is better, but if the entire shoe is stiff--doesn't the shoe sole become the pedaling platform?