Muay thai or bjj?

First, if you are into "Brawling" you shouldn't start training until you grow up. There are few things worse than a trained fighter picking on people that aren't trained. All of that said, if you're tall choose Muay Thai, if you're short pick BJJ, if you're an average height, say 5'9" - 5'11" either of them will do fine. If you plan on competing in mma someday you'll need to do both. Start with BJJ because if you're a fish out of water on the ground people will just take you down and submit you i.e. Mishima in PRIDE.