Aluminum has a low specific heat factor and conducts heat well, which means it heats up quickly, but it also means that it cools down quickly as well. Plastic is an insulator and does not conduct heat well. While this may seem like a valuable trait, plastic tends to hold on to the heat it's subjected to for a longer amount of time, dissipating it slower. Whichever material you choose, if you want to keep the intake arm from soaking heat from the engine bay, wrap it with some ThermoShield.