The 12-inch diameter pizza for $12.50 or the 20-inch diameter pizza for $17.50.

20 inch

Step-by-step explanation:

the 12" pizza for $12.50 works out to 11 cents per square inch and the 20" for $17.50 works out to 5.5 cents per square inch. So, basically, you're getting twice as much pizza for only $5 more if you go the route of the 20