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Well, if you want to look at it from a heat transfer point of view, aluminum is a much better conductor of heat than any plastic. The rate of heat transfer is pretty much proportional to the thermal conductivity of the material. Plastic ranges from about 0.1-0.3W/m*K and aluminum is about 200 W/m*K. Therefore the aluminum case will conduct heat about 2000 times faster than a plastic case.
The better question though is: How much heat is actually going to the structure of the case? My case has one fan brining air in, two ports in the back that can allow air to come in (no fan) and two exhaust fans (top and back). Most of the heat generated is from the power supply, processor, cards, and drives. Heat can get to the case itself by conduction through the motherboad and parts of the case, but none of that is directly touching the exterior of the case and therefore isn't very efficient. The exterior of the case can also accept heat from the surrounding air that is heated by the heat sources, but that isn't very efficient either.
If the air is well circulated, the primary way for heat to escape the case is going to be your fans. Having a system that is set up to maximize circulation within the case and an efficient intake/exhaust system of fans will make a much bigger difference than what material your case is made of.
Another thing, heat has always been an issue with computers but only recently have they started making aluminum cases for PCs. Although today's equipment might run hotter, the material the case is made from has never been an issue.
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