This guy used the follow to figure it out:
Area of a circle = pi*radius²
Two 8″ pizzas = 2*pi*(4)² = 32*pi square inches
One 12″ pizza = pi*(6)² = 36*pi square inches
In short, you are better off with the one large 12″ pizza over the two medium 8″ pizzas.
but what if your favorite part is the 1" crust?