Tai chi can be best described as "mediation in motion.
" It is more slow-moving than tae kwon do and focuses more on internal energy development whereas tae kwon do is more external.
However, that does not mean that tai chi itself will not provide a workout for you.
It may not be as strenuous or physically demanding as tae kwon do, but you will see some physical benefits out of doing it.
In general, the movements will work your core, back, legs, and arms.
Though it looks slow from an outside perspective, some of what you may be doing might not come so easy.
Tae kwon do on the other hand is more "hard" and external with its phsyical movements.
You will be doing various punches, kicks, techniques, and forms as you go through the curriculum.
Sparring, I believe, is also another component of it.
Doing that for a while will have an effect on strengthening and conditioning your body for learning and adapting to the material.
It really depends on what you want in terms of "fitness and exercise" so it might be better to clarify that to get a better answer.
I started doing both kung fu and tai chi at the same time at my school, but many people choose one or the other.
Some people who do only tai chi do it just to move around (who tend to be older in age as well).
I have seen people who choose that route improve their posture, joint movements, and some flexibility.
People at my school who jump right into kung fu will also develop more physical stamina along with overall body strength.
Considering the difference between those two styles, what I would do is try to visit each class and observe it or try it out if you can.
Stay with each one for some time to get a better feel of what you will be doing and how your body will respond.
However, I don't think you should be averse to choosing one over the other (maybe not now, but perhaps later) since neither one should impair what you do in the other art.
You may find them to be complementarily beneficial (over time once you develop a deeper understanding of your physical movements and what each art entails).
martialarts.stackexchange.com/users/15/Matt Chan
Permit me to reinforce what Matt Chan states.
Style (tae kwon do or tai chi)
(in my opinion) ought to be a tertiary selector.
I believe that I'm not alone in stating that long term success/happiness in martial arts derives first from the instructor, second from the classmates and only third from the art.
You may want to read one of my prior prolixities on the subject of how to choose a school
So my advice is to visit both classes, observe and then choose.
My daughter is a shodan in Tomiki Aikido; when she went to college we contacted the aikido group there, and she joined for a few classes.
Total bust.
The instructor didn't show up, the classes were taught by a junior student with insufficient understanding of either aikido or teaching.
She switched to isshinryu karate.
My current Tai chi teacher teaches two classes.
In our class we have to strap on pads for some exercises - full contact tai chi.
In his other class, they never ever discuss the martial applications of tai chi.
Tai chi can be both practical and a good workout.
Because you said that you're primarily interested in fitness, I'll only mention briefly that the push hands exercises in tai chi have been quite beneficial for my aikido.
From a fitness perspective, even after 10+ years of aikido, tai chi has given me stronger stabilizers, and I walk out of class with a sweat after every session.
(I'll probably sweat less as I get better at push hands).
martialarts.stackexchange.com/users/457/Mark C. Wallace
If you're sedentary in a new city and looking for exercise, the best school to join is whichever one actually moves around vigorously that you will enjoy and stick with .
Whether it's Tae Kwon Do, tai chi, yoga, lifting weights, or soccer doesn't particularly matter.
Whether or not you sweat matters.
Whether you like it and keep going matters.
However, it's very likely that practice at the tai chi school involves less vigorous activity than your average yoga class.
Most tai chi schools practice the form as a slow dance, with maybe a small amount of time spent doing very soft and compliant partner drills.
It would be better than nothing at developing fitness, but certainly not a complete or optimal solution.
Try the class and see how sweat-inducing it is.
I'll bet that the TKD school makes you sweat harder.
martialarts.stackexchange.com/users/347/Dave Liepmann
If you are looking just for exercise and fitness, martial arts in general is probably not a good choice.
Crossfit, P90X, walking, jogging, biking, swimming will work better.
Most "lifers" don't rely on class for conditioning, and will tend to practice and condition on their own.
Tai chi is good if you want to start correcting your posture
but so can the milder forms of yoga.
Dancing also works well, and you probably have a lot more fun.
martialarts.stackexchange.com/users/65/Ho-Sheng Hsiao
From personal experience, I would say it's essential to develop a general understanding of martial arts from other schools before beginning with Tai Chi.
Not only because it is the art with by far the highest number of fraudulent teachers, but also because you just won't be training it properly unless you understand how it works as a martial art.
I train currently with an honorary member of the Chen family, and even in his classes I'm seeing a great number of people using sweeping motions where there should be piston motions just because they only think of the postures they need to end in rather than the purpose of the movement.
martialarts.stackexchange.com/users/8681/Alistair Sweeting
I recently moved, and I found there are two classes within walking distance of my home. One is tai chi and the other is tae kwon do. I am in my thirties, have led a sedentary lifestyle so far. I'd like to join one of these two classes, for exercise and fitness. Any thoughts/advice?  I am not overweight and don't have any physical ailments though I am slow and weak.
For a total beginner, is tai chi a good start?
martialarts.stackexchange.com
<tae-kwon-do><tai-chi>
martialarts.stackexchange.com/questions/1485
martialarts.stackexchange.com/users/716/user187809
For just exercise and fitness, martial arts in general is probably not a good choice. It is essential to develop a general understanding of martial arts from other schools before beginning with Tai Chi. Try to develop an undrestanding of martial arts first as it's probable not the best to start with first if you are just looking for exercise and fitness.
Try to develop an undrestanding of martial arts first as it's probable not the best to start with first if you are just looking for exercise and fitness.