Moly or Plasma Moly Rings?

They are the same thing, just more marketing noise. All basic rings are cast iron. In more racey performance rings you can improve on that, but for what 80% of us would use, it's an iron ring, that has a coating on the face of the top ring (where the ring meets the cylinder wall). That coating might be chrome plating (very hard, not for a street or drag race engine, really) molybdenum (called moly for short), or plain iron faced. All take a unique cylinder finish, so your machinist should tell you what the cylinder is honed for, and you buy that, and follow that prescribed ring seating process.