i teach both Muay Thai and Wing Chun, so i feel i can shed a little light on the subject.
What i love about Muay Thai - conditioning, conditioning, conditioning! this inherently is what makes it so effective. it pushes you through physical pressure and mental pressure which shapes the individual into becoming physically fit and mentally tough.
What i hate about Muay Thai - conditioning, conditioning, conditioning! it's heavily based on athleticism which means it's good when you are young but as you get older (i am 46, i started Muay Thai at 20) it becomes harder and harder to maintain the very essence of what makes Muay Thai great...the athleticism. i compare it to a Lambroghini. it's super fast and a high performance vehicle. but to keep it high performance requires constant maintenance. you cannot let a Lambroghini sit stagnant too long or else it performs like crap. you cannot use regular oil. it requires special fuel.
What i love about Wing Chun - to create force, it is based on the mass and not so much the acceleration or speed. Wing Chun is also based on leverage, which is the crucial ingredient to do little but gain alot. In other words, it shares the same concepts as the Grappling Arts. Because of this, it allows the person to train at a high level even at a much older age. i compare it to a Steam Roller. it's mass is heavy but moves slowly.
What i hate about Wing Chun - the wing chun community. ugh, i can't stand their pettiness. they are so divided and can't even get along. they agree on nothing and fight over the most idiotic things. the truth is that most couldn't fight their way out of a paper bag! The vast majority of Wing Chun practiced today is devoid of real physical and mental pressure with a non compliant individual. without that heavy, consistent pressure testing, it ends up becoming a crappy version of short arm kickboxing.
so, the bottom line is, if you are younger in age, or able and willing to perform the constant maintenance, then i would choose Muay Thai. if you are older and want a martial art with longevity, then i would say Wing Chun, but only if you practice it with a Grappler's attitude and training methodology.
Here is a video interview i did a few years ago. Just a FYI, i had nothing to do with the filming or editing of this video. As a side note, i live, teach and train here in the los angeles area and love to train and workout with fellow martial artists. i'm often at open mats all around town.