How they decided these materials was through an extensive process of material testing (this is one of my MANY jobs at work). You can test different material properties with very simple set-ups that will tell you the following and more:
Core shear, skin shear, core crushing, overall tensile strength, overall shear strength, modulus of elasticity, poisson's ratio, bending strength, as well as a lot of others.
However, these tests take time and a lot of patience and control. For one, you need to make sure each test sample is EXACTLY the same!! Two, each test must be run under the same conditions. Three, you need to know a little prob and stats to reduce the data into a workable allowable.
Overall, the tests can be very time consuming, but they will get you the comparison that you need... or you can just trust Rutan on his original design and stick as much to it as possible...