For anyone who is confused on how the gyroscopic effect works:
(Quoted from Blake_T)
Discs with wider rims have greater mass distribution on the outer edge of the disc and will have greater gyroscopic tendencies than discs with narrower rims and more mass towards the center of the disc. That is, a wider disc will be more difficult to turn over, but will be more apt to continue to turn once it has been turned. A wide disc will also be more likely have a strong fade at the end of its flight as the disc slows down. Also, the wider the rim, the more the disc will change its flight characteristics due to changes in disc mass.
A wide rim disc holds a large percentage of its mass on the rim. If you take two discs of the same wide rim mold, one heavy and one light, they will have roughly equal mass on the flight plate (it can only get so thin without collapsing) and the majority of the mass difference will occur on the rim. A disc with more mass on the rim exerts greater gyroscopic forces than a disc with less mass on the rim (assuming they are thrown the same). Discs with narrower rims carry less mass on the rim and more on the flight plate so the effects of mass changes will have smaller effects on disc stability between different discs in that mold.
Higher end plastics have greater density than lower grade plastics. The result is that a disc of a particular mold run in high grade plastic will have a greater percentage of mass on the rim than a disc run in low grade plastic. Thus, higher-grade plastic discs will hold stronger gyroscopic properties and will yield increased high-speed stability and more low speed fade than their lower grade plastic counterparts.