This guy used the follow to figure it out:
Area of a circle = pi*radiusÂ²
Two 8â€³ pizzas = 2*pi*(4)Â² = 32*pi square inches
One 12â€³ pizza = pi*(6)Â² = 36*pi square inches
In short, you are better off with the one large 12″ pizza over the two medium 8″ pizzas.