What material is good for the car? The big contrast between the iron tank and the plastic mailbox

It is reported that the iron tank has been gradually eliminated in developed countries, now the plastic tank used in most of the developed countries in automobile production, but the country is implementing GB7258-2012 vehicle driving safety "technical conditions" above and does not specify the material of the automobile fuel tank. The introduction of the standard explicitly pointed out that this standard provides for the safety performance requirements and test methods of gasoline and diesel fuel for automotive fuel tank. This standard applies to the metal fuel tank and the plastic fuel tank of the M class and the N class car." We used cars in the national standard on the M class car. Since the national standard has recognized the legitimacy of the metal tank, then the iron mailbox and plastic for the truth in the end is what?