Kung Fu vs Muay Thai: two stand-up fighting styles that derived from similar cultures of Far East. Both of them utilize kicks and punches while Muay Thai also make use of knees and elbows. Kung Fu is an ancient - around 2500 years old - martial art and a Thai boxing is a younger - around 600 years old - martial art but also which rich history and traditions. Let's start.
Kung Fu, same like Karate, is generally known to a wider audience, especially thanks to famous adepts and actors like Bruce Lee or Jackie Chan. Muay Thai may be a bit less known but it's quickly getting traction since Thai Boxing is a weapon of choice in major MMA competitions and it is also going for an Olympic recognition.
Both systems are rooted in a weapon based martial arts: Wushu for Kung Fu and Krabi Krabong for Muay Thai respectively. I will argue that Thai boxing came out on top as a more useful "no weapon" fighting style than Kung Fu.
Same as Karate, Kung Fu is built around forms while the main focus of Muay Thai is sparring. Practicing forms and poses doesn't prepare a fighter for a real battle with skilled opponent and it is not a very effective way to learn either. Thaiboxers on the other hand practise their skills by sparring. This allows them to learn more quickly and, which is even more important, to obtain skills that are useful in real fight.
I have already pointed out forms - kata - weaknesses in Karate vs Muay Thai article. Same applies to training methods which, in Muay Thai, are based on practising with the real opponent instead of punching the air.
In the 70's a famous match between 5 Kung Fu and 5 Muay Thai fighters took place in the Lumphini Stadium in Bangkok. Chinese martial arts adepts came to prove themselves against Nak Muay. Thais won 5 out of 5 bouts. All of them in the first round by KO or TKO. They even agreed to fight bare-handed at the request of a Kung Fu practitioners who wanted to take an advantage of their fist techniques. They were helpless against Muay Thai and its techniques and fighting philosophy. As a matter of fact, there was no single recorded competition where Chinese martial artists prevailed against Thais in the ring.
Quick warp to present time: Max 3 2013 Muay Thai competition in Zhengzhou, China. Muay Thai fighters from all over the world and a lot of Chinese Thai Boxing adepts competing against each other. The Chinese fighters have learned their lesson and started training and adapting Thai Boxing fighting style. Who knows, it may evolve into some form of a new interesting hybrid style. Why? Because Muay Thai is very effective. Thai boxers are hardened by years of sparring and fighting. There is no place for complicated forms but for straightforward techniques and body conditioning only.
I have recently watched this fight between Buakaw Por Pramuk and Yi Long "Shaolin Monk". It is very interesting bout to watch, especially the beginning of the fight. It also shows why proper body conditioning and great stamina are so important.
Also read Muay Thai versus other martial arts (Karate, Taekwondo, Capoeira and more) post.