What should I take Muay Thai/Kickboxing, or Mixed Martial Arts?

It depends on you. Remember though, most fight end up on the ground and if you don't know how to fight there and you end up on the ground with someone that does(cause they'll be able to take you down even if your a good stricker) you'll probable loose.

Mauy Thai and MMA are both sport styles however, and a real fight on the street is not a game of a contest it could be a matter of life and death..

Boxing is mainly like comp but if you want to learn self defense go for mixed martial arts.

For streetfighting, the best thing to stick with is Jiu jitsu. However, between the two you want to take, i would say MMA. Freestyle MMA (just walking into an MMA gym with no martial arts background) is really bad if you want to be an MMA champ. If you want to beat up your average nobody, however, go for it. Good way to learn the basics in (usually) BJJ, Muay thai, wrestling, and boxing. Also good way to stay in shape.