Theravada is the most basic of forms, the word means "small vehicle." There are many forms of Mahayana, (which means "large vehicle") so if you choose this path you might have to do some more research. I suggest Theravada, keeping it simple.
Theravada tends to be more orthodox in its teachings, and is a little more strict in terms of whether or not monks are allowed to accept money, and a few other little things. I believe Mahayana is what the Dalai Lama is, but there is also another school that you should check out, if you haven't already, the Vajrayana school. However, having background in most of the Buddhist schools of thought, I haven't found one to be superior to the others in any way. I like the philosophies and the teachings of Buddhism in general. You don't have to believe everything that you learn, but like the Grateful Dead said, "Believe it if you need it, if you don't, just pass it on."
It was during my study of Buddhism that lead me back to Christ!
I was reading "The three pillars of Zen" and realized that any religion that thinks you can Neel down in front of a Idol and say ambit's or hail Mary or anything else and be absolved of your sins is HORSE HOCKEY.
So I started reading about the first Buddha "Siddhartha" and I found the similarity's between his teachings and those of Christ so similar though he was born 600 BC.
I truly believe that he was enlightened by the same Holy Spirit
that enlightened Mosis.Jesus.and Mohammad and the problem is not in what they taught but is in all the bull everyone that came after them tried to add to there teachings.
So now I meditate and follow the middle way as taught bay Siddhartha and pray to and except Jesus as my lord and savor. A real good book to read is "Living Buddha, Living Christ." by Thich Nhat Hanh he has written many books and is one of the most Christ like men I have ever heard speak!
I practice Soto zen because I think it's a Mahayana style that is pretty close to the Buddha's original teaching. (I live in Japan where Mahayana is the dominant practice.)
If my only choice were of the two, I would choose Theravada. It is closer to what Buddha taught, his Dhamma.
Dhamma is the teachings and only the Dhamma, in the old day we were called Dhammafollowers, not Buddhists. And there was no such thing as Buddhism or sects (Theravada, Mahayana, etc.) Buddhism was called by Buddha, Dhamma Vinaya the doctrine and discipline.
Man has changed many things, but the teachings of the Buddha can be found readily in the Pali texts. Go to the source and the best source for the Dhamma is the Tipitaka.
The Pali Canon is the complete collection of the Dhamma discourses, preserved in the language of its composition. It is called the Tipitaka or "Three Baskets" because it includes the Vinaya Pitaka or "Basket of Discipline," the Sutta Pitaka or "Basket of Discourses," and the Abhidhamma Pitaka or "Basket of Higher Teachings".
In China, people eat food with chopsticks. In Morocco, people eat food with their fingers. In England, people eat food with knives and forks.
How people eat food turns out not to be very important. What's important is that people get a full belly and have the energy to help themselves and others.
There are many types of Buddhism and they all help people get a "full belly" -- that is, it's possible to attain enlightenment by following the teaching and techniques of virtually all Buddhist traditions.
Since you asked for a suggestion, I suggest that you find a teacher and community that seems suited to you and then practice with that group, in their tradition, for the rest of your life.
If you live in a place where you have several Buddhist centers, visit each of them and take the time to understand how they teach. Do you connect with the teacher? Does the community feel healthy? Stay alert and you will find your way.
The links below provide access to thousands of temples and centers worldwide. If you have more questions, you can email me. Good luck!