The basic design of thermal mugs, whether they are plastic or stainless steel is the same – double wall insulation with a lid to seal the top. But even though stainless steel versions cost more than plastic versions, in the long run, they are the best deal for travelers. They can also be made of ceramic or glass, but those are generally designed for household use and not for travel due to the possibility of breakage.
The main benefits of a stainless steel mug are durability and better insulation. Steel is tough, and although plastic mugs can take a beating, a steel mug can last much longer. This is especially true when compared to the more brittle, hard plastic mugs that are sometimes used to display stylish designs and finishes. Although messy, dropping a steel mug of coffee will not generally cause any damage.
There are three basic types of insulation used; Air, foam or vacuum between double walls. Plastic versions usually use either air or foam. Both work acceptably, but air or foam is not as effective as vacuum insulation. More often than not, steel mugs use vacuum insulation which can keep beverages hot for a longer period of time. This can be quite important especially on long (and cold!) Morning commutes.
Beyond the two primary benefits, there are a few other benefits that stainless steel thermal mugs have when compared to plastic mugs. Plastic, being slightly softer, is more difficult to clean. You can use tough cleansers (making sure they're non-toxic!) On steel, but the same cleansers will scratch and damage plastic. For the same reason, plastic mugs tend to retain the flavors of the beverages that have been in them. This is not really a problem if you just drink regular generic coffee every day, but if you use the mug for different beverages, it can be.
When you look at all the factors, durability, ease of cleaning, effective insulation and better flavor, the stainless steel thermal mug is by far the best choice, even though it can be a bit more expensive. The only real benefits of plastic mugs are the lower price in the short run (which can be a big benefit if you lose your mug frequently!) And that they are slightly lighter than stainless steel mugs.