(excerpt from Wayne Dalton)
Most polystyrene insulated doors can be thicker but have a lower R-Value than polyurethane. If you are looking for thermal efficiency and a high R-Value, do not be fooled by the thickness of the door!
A polyurethane insulated door might look thinner but actually offers a much higher insulation R-value. That is because of the way the insulation expands during the manufacturing process to tightly fill the cavity between the two steel panels as it bonds to the skins of the door.
Not only does this make the door better insulated, it also makes the door incredibly strong and helps lessen noise while the door operates.