Hi There!

  I would say that between those two, everything depends on the instructor.  Much of Brazilian Jiu-Jitsu in the beginning year or two is done on the ground, and if a person attacks you in a parking lot or a group of people attacks you in a bar or something, you don't necessarily want to go to the ground yet.

  However, Muay Thai requires an incredible amount of devotion and physical fitness to master to the point of being able to use it for self defence.  

   If you're looking specifically for self-defence, and you want something you can use pretty instantly, I recommend Jujitsu or Aikido.  They're excellent, soft martial arts styles that don't require much conditioning.

   But, if you live in an area where your only options are Brazilian Jiu-jitsu or Muay Thai, I recommend going in and talking to the instructors each individually.  Let them know that you are looking for "practical self-defence" training - the word practical is an important key word for martial artists.  It means they won't just teach you the "kata" or pretty forms of all the techniques, they'll teach you the down-and-dirty make-it-work forms as well.

   Good luck to you!  I hope you find that the martial arts becomes a valuable and rewarding part of your life.

  -K