JT,
Hardcover Model, doubt you'll ever see it referred as that in an advertisement, is a basic model, just one grade over the Simple Model, that contains a cyclonic water separator. The Cyclonic water separator is a rectangular aluminum block, mounted vertical on the front side of the compressor, that has a red plastic cap with a pull ring. Simple Model will not have a water separator, and you'll want one. Hardcover Model has a plain glass face on the pressure gauge. Set Pressure and Auto Stop Models have a red plastic knob on gauge face, used to set pressures. These features cost extra, and you don't need them. You can tell by the price, too. Simple model cheaper, Set Pressure and Auto-Stop higher than Hardcover. Hope this helps, WM