Answer:
The best answer for this would be the medium
I am not to sure on how to explain it besides you'll be getting more outta the two mediums than you would be getting with the single large. If there are 16 slices then the box will be big but slices very thin and what not. Medium boxes with each will have an average size slice and with two boxes then ya will in my perspective have more.
.....I order pizza way to often