In my opinion, you can't really compare the two. Muay Thai is bred to be in the ring. All muay thai fighters will have superb conditioning and will use effective tactics such as clinches and low kicks and most will have great stamina.
Taekwondo itself was usually for self defense and is not designed to last long periods of time in the ring. Its usually designed to finish someone in a few moves if you look at poomsae or techniques in taekwondo,