As long as it is properly constructed and installed, either work fine. This is another one of those topics where experts will line up on both sides of the question. Additionally, we have had about an equal number of customers state that:
What it seems to depend on most is whether you personally have ever had a bad experience with one material or the other. The question is almost moot however when you get into big boats with big fuel tanks. In those situations, it is highly unlikely that you will find a plastic tank that will work. Typically, to get the substantial weight of a large tank full of fuel in the right place in the boat, it needs to be specifically designed for that boat.