What are the differences between Zen Buddhism and Tibetan Buddhism?

I am primarily interested in following the teachings of Mahayana. Mahayana refers to the path of the Bodhisattva, a person capable of achieving Nirvana, a state in which the practitioner is released from the effects of karma and the cycle of death and rebirth, but instead chooses to save other beings from suffering. Zen and Tibetan Buddhism are two different schools of Mahayana. What is the essence of these two paths, and how do I determine which path to follow in order to become a Bodhisattva?