When moly was first used in piston rings, a groove was cut into the center of the ring and moly was applied to (filled) in this groove. Then the OD of the ring was finished. These rings were usually grey iron. In most cases, moly-filled rings still utilize grey iron base material for the ring.
In subsequent years, ductile iron has been used for top rings in high performance applications. As stated in a previous post, plasma moly is sprayed onto the face of the ductile ring, under very controlled temperature, and then the final OD of the ring is finished. Just about all high performance rings now use ductile iron or steel for the top ring, with plasma moly spray.
Filled moly and plasma moly have about the same physical characteristics and will work just as well in most applications. For severe usage (that's most of us), ductile iron or steel top ring with plasma moly face will survive much better.