Without getting in to a big debate I'd like to correct a few misconceptions previously mentioned in this thread. Really only to 'clear things up because I often hear these ideas going around.
Chi is the result of proper circulatory response through LACK OF TENSION. Had you done some research, you would now see how crazy this discussion is.
Taiji is not about 'lack of tension', Its about the correct tension. Ji is the result of the correct body position, correct muscle activation, sensitivity to individual muscles, grounding and intention.
External arts are for people developing muscle, strength, and agility.
INTERNAL arts are for people developing health and maximum effectiveness through efficient energy usage.
Maybe in the west, you could say this is a good stereotype. But This is not true of martial art based Taiji. People seem to think that all Taiji is practiced slowly, however Taiji is extremely explosive (how would it ever be an effective martial art if it wasnt?)
As for external martial arts being for people developing muscle, strength and agility I would disagree. Agility yes, But in my 13years in Karate I have never seen an example of external arts making you stronger. As you well know strength comes from iron, not the repetitive motions of a martial art.
Modern methodology of weight training causes muscle tension which is counterproductive in Taiji. Taiji requires complete lack of tension to be effective.
I have found that my improvement in strength over the last 3 years had improved my Taiji greatly. I have been studying Taijiquan (Tai-Chi-Chuan) for 8years now and I have never had such a rapid boost in control and I now find it easyer to activate a particular muscle. Yes you loose some sensitivity but you gain so much as well.
Weight training is great for Taiji.
Taiji is one of only 3 internal systems, all other martial arts are external.
I'm not quite sure where you got that from. Yes there are more external systems than internal but to limit them to only 3 is a bit misleading. No one will ever be able to count all the styles as many are still (even today) secretive to families and small groups. However Just to broaden the spectrum a bit (as talking about the difference between Int and Ext MA would take hours) here are a few martial arts that you may not know are classed as internal.
More than 2/3rds of Gong fu styles
Wado Ryu Karate
Wrestling
Sumo
Judo
Akido
Hapkido
Very very briefly - An Internal style is one which uses sensitivity to connect with the opponent, feel where they are going, and use their own force against them or take them down using their own mistakes.
External MA are martial arts which do not connect with their opponent before attacking. External MA will use the eyes and often go against a direct force where in Internal guy would flow with it and alter the direction of that force.
Back to the question. Yes Taiji is a great tool. However it is (I would class) an advanced martial art and you would have to be very special to be able to apply it to self defense within your 1st few years of training. But dont stop, cos you'll find many uses for it as you get better. If you want to learn self defense then do Taiji and another art that you enjoy. I wont advise a particular Gong fu or karate, just go with a class that you like.