The 12 inch pizza is the better deal. It's all about surface area.
Formula for figureing out the area of a circle is (PI*R*R) I forget how your supposed to make it look
Since they measure a pizza based on edge to edge you have to take half of that to get your R.
so in the case of the 6 inch pizza each one should give you an area of 28.26 square inches.. or the 2 of them 56.52 square inches. For $10 they cost you $0.1769 per square inch.
For the 12 inch pizza you get an area of 113.04 square inches giving you a cost of $0.1061 per square inch....