there are 3 kinds of cases (FCC approved), steel, aluminum, and plastic (with steel frame).
steel: tried and true, generaly lasts forever, heavy as hell
aluminum: light as hell, fairly new to market.
plastic: only the exterior is plastic (panels), the frame is still metal (usually steel). there are metal sheets that are used for EMI shielding.
the plastic panels help dampen the sounds produced inside the case, and produce less sound themselves than metal panels.
--
each kind of case differs in quality from manufacturers. i happen to have 2 steel cases. AL is too expensive for me. i have nothing against plastic cases, i just don't happen to have one. there are some good ones.
each have their own quirks about modding. plastics melt, metals are "hard" to cut...
notice i did not talk about those fiberglass cases, they are not fcc approved due to lack of emi shielding.
--
in terms of heat dissipation, AL does do a better job, but it's neglible if you have fans placed correctly. cases themselves are used as a heat spreader (not quite a heatsink). and design for air cooling differs from case to case.