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?
From my experience, the biggest diff is in the amount of rational explanation.

Tibetan teachers and texts are much more inclined to giving a step by step logical rational reasonable conceptual framework. Zen teachers and texts speak to direct experience, using spur of the moment analogies and play on words.

Judging by the very fact that you asked this question, you seem to be inclined towards the Tibetan style. (It seems to me if you were a good Zen material, asking this kind of question would not even occur to you. You would not trust anyone's opinion and would be inclined to try both and find out for yourself.)

And judging by how you mentioned "essence" and your practical tone, I suspect Kagyu may be a good lineage for you.

If you are interested in differences between Tibetan schools, you may want to see this answer.