These two are known to be the two types of plastic piping. PEX is also known as cross-linked polyethylene and PB Piping is Polybutylene. The main difference between these two types of plastic piping is the way that they are created. PEX has materials that are better for processes that involve the use of metals.
PEX is also considered to be the better option when dealing with corrosive materials and fluids that are cold. PB Piping is one of the first plastic pipings that were used but as time went on, people realized that it is not strong enough to withstand high pressures and high temperatures. This explains why this is not allowed to be used in some countries anymore.