Tai Chi is a Chinese art designed to protect oneself from unarmed and armed attacks and illnesses. It is both a martial art and a method for preventing and treating illnesses. Information about its history and concepts can be found in the article "Are You Really Learning Tai Chi and Is It Effective for Stress?" http://martyeisen.wordpress.com/2010/05/09/9/
The main divisions of modern Qigong (Chi Kung) are: Spiritual, Medical, Martial and Athletic depending on the main goal of the practitioner. However, there is an overlap between these branches.
Medical Qigong is a branch of Traditional Chinese Medicine (TCM). TCM postulates that health is the result of smooth Qi circulation, without accumulation or deficiency in any part of the body, while disease is the result of poor Qi circulation. Once the flow of Qi is balanced, the body tends to heal itself.
Here Qigong will be used to denote Medical Qigong for preventing and treating diseases and will not include Tai Chi, which is also a form of Qigong. More information about Qigong can be found in "What is Qigong" at www.selfgrowth.com/articles/Eisen1.html .
If you want to learn classical Tai Chi you must find a knowledgeable instructor. Many classes only offer shortened, altered forms that are supposed to be for health purposes only. Many of these so-called Tai Chi forms are not taught according to Tai Chi principles and are some sort of new-age dance. You can save money and probably get the same health benefits from slow walking and waving your arms. Even if you find an instructor that teaches a classical long form and you want to learn self-defense, investigate if 2-person Tai Chi martial art forms are taught and not self-defense based on Karate or some other martial art.
Many health benefits have been ascribed to the practice of Tai Chi. However, there are a few problems with these studies. The scientist conducting these studies may not be an expert in Tai Chi and so selects an incompetent teacher for the subjects. The form may have been altered by the instructor and so another teacher may not teach the same, exact form in another study. Sometimes the sample size is too small. If the subjects are required to practice at home some will be fanatic and others barely practice and accurate reports of practice times are difficult to obtain. Finally, Tai Chi, even shortened versions for health, require years of study before the subject is really doing Tai Chi. Hence, the studies should be called the effects of trying to learn Tai Chi. More long-term studies are required to verify the beneficial effects of Tai Chi.
Clinical trials have shown that Qigong is helpful in about 200 diseases, even more diseases than Tai Chi. There are many different forms of Qigong. Some are classical and others are made-up or modified by a teacher. The criticisms given above for Tai Chi research apply to Qigong research.
Learning Tai Chi takes self-discipline to practice daily and tenacity not to give up. Those who have studied arts, like dance or music, that require constant practice are more likely to succeed in learning Tai Chi. Tai Chi requires more coordination than some forms of Qigong. A good memory is also helpful. Nevertheless, learning Tai Chi only for health requires years of study.
Seniors interested in preventing or treating illnesses should consider learning Qigong rather than Tai Chi, especially if they have memory or coordination problems. There are health forms of Qigong that are much easier to learn. For example, a Qigong method for losing weight can be learned in about a minute. The hard part is to practice it before every meal.
For life threatening diseases use Qigong. You may die before you learn Tai Chi. Some hospitals have drop-in Qigong classes. Such classes are not useful for treating serious diseases. Usually, you are not informed that you must practice for hours every day. Find a specialist in medical Qigong who can give you a TCM type of diagnosis and construct an individualized Qigong protocol based on the diagnosis.
Even in China, it is difficult for patients to practice Qigong for hours. Patients join a social Qigong group or are placed in a hospital.