The basic design of thermal coffee mugs, whether plastic or stainless steel is the same with double wall insulation with cover to cover the top. The stainless steel version costs more than plastic versions, and in the long run. They are the best deal for travelers. They can also be made of ceramic or glass. But they are generally designed for home use and non-travel due to possible damage.
The main benefit of stainless thermal coffee mugs is its better durability and insulation. The plastic cups can take and beating. But the stainless mug can last much longer. This is especially true when compared with more fragile plastic cups, difficult to be used sometimes to display elegant designs and finishes. Despite the mess, dropping a cup of coffee hard does not usually cause damage.
There are three basic types of insulation used; air, foam or vacuum between double walls. Plastic versions usually use air or foam. All of the works are received, but air or foam is not as effective as vacuum insulation.
Outside of the two main benefits, there should be compared many other advantages of stainless steel thermal coffee mugs with a plastic cup. Plastics are a little lighter, more difficult to clean. You can use a tough purifier (to make sure it is not toxic!) In the steel but it will the same vacuum cleaners, plastic damage.