Brazilian Jiu-Jitsu is primarily a grappling martial art. You'll learn takedowns, throws, and ground grappling, as well as control positions, escaping, and submissions such as arm locks and chokes. There is no striking in Jiu-Jitsu.
Muay Thai/Kickboxing is a stand-up striking art. We say that it uses all of the body's 8 weapons: punches, kicks, knees, and elbows. We also teach proper footwork and timing. It's the most effective striking art that there is.