I can only base this off of my experiences as i have trained both kickboxing and Muay Thai for many years.
1. Kickboxers on average are volume punchers, punches in bunches, combinations, etc.... They also have more western style "slick" boxing in terms of punch defense (head movement & footwork) in contrast to Muay Thai where usually you would stand your ground & just shell-up to block or parry the punches and return fire or trap the arms & try to push forward to initiate a clinch battle.
^not to say Muay Thai guys can't learn this too, it's just that kickboxers focus on defending and using punches more because there's no clinch & because their sport is basically all punches and kicks which are scored equally, so being heavy volume punchers would benefit them alot in terms of scoring, while punching alot in Muay Thai doesn't necessarily help the scorecards.. So naturally your average kickboxer tend to be better 'boxers' than Muay Thai fighters, of course this isn't always the case, but it is alot of the time.
2. In kickboxing a variety of kicks are taught. Aside from your bread and butter roundhouse kick and push kick, kickboxers also regularly practice "less practical" kicks like spinning kicks & jumping kicks and such. Whereas Muay Thai fighters stick to the basics (roundhouse & teep to the legs/body/head) and spend alot of time perfecting them with lots of repetition, nothing fancy, just basic kicks that have a high percentage of landing on your opponent.
3. In kickboxing there are more preset combinations during padwork, in my school we did alot of preset combinations on eachother during padwork and partner drills, i think it has alot to do with the karate influence. Whereas in Muay Thai the padwork is less robotic and more interactive, simulating a real fight, you pick your shots when the pads are flashed or if your pad holder is real good you can just throw anything and he'll catch it, sometimes there are combos, but they tend to be very basic.
4. Muay Thai has lots of clinch wrestling which includes knees, elbows, throws & sweeps, kickboxing might have a little depending on the school, but in most cases they don't clinch wrestle. They might throw knees, but usually they are long outside knees without the clinch. They purely strike.
5. Muay Thai is Muay Thai (Kicks, punches, knees, elbows, clinch wrestling), whereas Kickboxing can take on alot of forms, some schools kick with the feet like TKD & Karate, some schools do mostly boxing with some kicks, some kickboxing schools teach low kicks, some don't (american kickboxing). Some have belt ranking systems etc...It's a huge clusterfuck, you can call almost anything "kickboxing" these days. Alot of "kickboxing" schools even call themselves Muay Thai even though they don't clinch, do padwork, or throw knees/elbows.
I say Just train traditional Muay Thai and learn some "real boxing" on the side to brush up on your hands a bit(you don't really have to if you're planning on competing strictly Muay Thai though, too much boxing is just asking to get clinched)....Kickboxing is so random and there are alot of teachers that come from Karate/TKD backgrounds or don't know shit about the modern kickboxing scene. Luckily my kickboxing teacher from the past knew his shit & trained guys like Maurice smith who fought guys like Hoost & Aerts, but it won't always be like that.
