Interesting crust question.
C=2πr
So 12" has 37.7" x 2, or 75.4" of crust
and 18" has 56.6" of crust, meaning if you don't like crust, you waste far more on the smaller pizzas, which proportionally have a greater amount of crust.
HOWEVER - if you like cheese stuffed crust, the smaller pie will give you greater cheese pleasure.