Brazillian Jiu Jitsu
I would go to both. The advantage of mauy Thai is superior clinch abilities,striking and takedown/s defence to an average early bjjer but all those skills become of no use against a experienced bjjer. They are really good arts both of them if you want a straight style vs style ill tell you the truth. If you had a pure mauy Thai fighter vs a pure bjj fighter the bjj would win more than half the time. I would combine them and try mma its abit softer than pure mauy Thai and that's where most injurys would occur.
What do you want to learn? If you were to cross-train in either Martial Art then you would have a good round combat capability in a fighting situation.
Beware of Mcdojos etc. because you want to learn the correct art.
id learn jiu jitsu, its pretty easy on the body.
Try both, you will choose the one you like better trust me.
i'm 14, 5' 6 and 110 pounds... which should i learn??? brazillain jiu jitsu or muay thai?
