Kung Fu vs Muay Thai: it makes sense to compare these two fighting style since they derived from a similar cultures and are both stand up fighting styles. Former is an ancient (2.5k years old) martial art and a Thai boxing is younger (600+ years old) but also has a rich history and traditions. Lets start.
more about KUNG FU
Kung fu, just like Karate, is generally known to a wider audience, especially thanks to a famous adepts like Bruce Lee or Jackie Chan. Muay Thai is a bit less known but this slowly changes since Thai Boxing is a weapon of choice in major MMA competitions and it is going for an Olympic recognition.
Both systems have roots in a weapon based martial arts: Wushu for Kung Fu and Krabi Krabong for Muay Thai. My point is that Thai boxing came out on top as a more useful no weapon fighting style.
Same as Karate, Kung Fu is build around forms while the main focus of Muay Thai is sparring. This allows Thai figthers to learn more quickly and, which is more important, to obtain a real skills that are useful in a real fight.