There is so much air moving through the box that conduction is minimal. The ideal box is polished aluminum facing the engine bay and an insulating coating on the filter side, but you get far more help from the reflective surface than the insulation. Here's what happens with a black plastic one: the radiant heat from the engine eagerly warms the plastic, and the plastic radiates the heat onto the filter. With a metal one, the radiant heat is reflected back to the engine, and most of the heat on the inside is from conduction. Temperatures will be very close, but aluminum with a polished surface will be cooler. If the shroud touched the engine, then you wouldn't want metal, but it only touches the air, so conduction is not a big deal at all. However: there is not a big difference between the two, even if temps are different. 99% of the job of the box is to ensure no hot engine air gets sucked into the intake. Use whatever material will make the best seal with the hood and engine bay, and that will be the best intake.
Cliff's Notes: metal is better at keeping engine heat out of the intake box, but it doesn't matter because the difference is neglible. Worry about the seal and flow of the box instead.
