Would it better for me to train stenght or speed for fighting?
As others have said, a bit of both, but definitely speed mostly. I have done Shotokan Karate for 8 years and Krav Maga for 1 year, and i am about your size but a bit lighter. I am in no way big and strong, though i am strong and muscular but i have not built up my muscles. Train speed and you will get further than if you train strength. Muay Thai and Krav Maga are both fantastic arts (despite Krav not actually being an art, but a fighting system), but you should go with Krav. Martial Arts should not depend on strength, but speed and power.