What's the difference b/w tai chi and tai kwan doa? And which one is better?

Tai Chi is a type of martial arts that uses fluid motions as its moves. The forms are done slowly and flow together. Muscle tone, strength, cardio excercise and flexibility are a big part of Tai Chi. Tae Kwon-Do is in a way like Karate with kick and punches, but the kicks are higher and there are fancier kicks in it as well. As with Tai Chi, strength, cardio excercising and flexibility are done in most classes, but in its own ways that interact with its style. The best thing for you to do personally is go to at least 2 of each style of school in your area, more if possible, less if there aren't many. Watch a class or two and decide for yourself what you are interested in. Be sure to not only watch the beginners classes but also the more advanced classes being that the advanced classes may be more intense and you'll see the overall idea of the classes taught at those schools. Also something else to look at, it Tai Chi is often matched with other styles of martial arts such as Kung Fu. Tae Kwon-Do Schools may be matched with judo or jujitsu. A school that is strictly one style may or may not be what your looking for.

tai chi is from china and tae is from korea, while tai chi is very spiritual as in yoga and tae is more of a striking and kicking butt

Tae Kwon Do is a hard art that uses lineal and circular movement. Tae Kwon DO means to Squash with the hand or the foot. To me Tae Kwon DO is Korean Karate.

Tai Chi is usually a soft art the focuses on balance, breathing and slow movements. Tai Chi is generally not taught as a self defense but if can be if you are lucky enough to find an instructor who knows how to use the movements for defense.

Tai Chi is more like Yoga. While it is a bit more on the martial arts side of things it it ment for toning muscles and learning how to control your body in a way that you are able to keep in shape through the use of flowing movement. Tai Kwan Doa is more like Karate where is is more fighting (yet not as tough as Karate). Personaly I think the Tai Chi is better for you, however it all depends on what you prefer as your work out. Good Luck, I hope I helped. Happy New Years!