If you can't do both then IMO I would go with the muey thai. The reason I say this is I feel that BJJ is more for 1 on 1 fighting. It doesn't help you on the street if you are facing two or more people. While you're grappling with one the other attacker could be kicking the crap out of you. At least with muey thai your standing up and not on the ground. I would deffinitely learn grappling though at some point in time.