OK folks, right out of a textbook, here's the lowdown on the inertia deal. The actual formula for an annular ring (imagine the rim without any spokes, since the biggest part farthest from the axle has the greatest effect) is:
I=Mass(Outer radius^2+inner radius^2)/2
This is including the effect of the thickness of the outer part of the rim. So it does go as the square of the sum of the radii. This means that the actual inner and outer diameter of the two wheels is what is important, not the 16" or 18" generalization.
I ran some really rough numbers and if you neglect any weight difference (which sounds close anyway), the inertia is about 30% greater with the 18. Even if the weight is different, it's the fact that the radius is squared that matters. This is also assuming a similar overall construction, but I feel confident in saying that the 18 will always have a higher moment of inertia.
Physics works! :p
