Hi mjk!
Sorry this post could not be made shorter without being of little value. I'll try my best to give answers to you as well as to explain what "yoga" means to me.
Chuck's comment in the other thread really is worth considering. Vipassana practice has a clear goal as everybody here knows. With Yoga, what's the goal? Enlightenment, for sure. But what else? ***
Patanjali gave a good answer in his yoga sutra: to avoid suffering that is yet to come. So while my teacher at the Malaysian Buddhist Meditation Centre who was taking medication to lower his cholesterol would take his illness as an example that no matter what you tell your body, it won't do what you want (here: lower the cholesterol level), so it's non-self and painful and decaying, a yogi with a different skillset, e.g. someone knowing asanas and kriyas (cleansing techniques), would most certainly answer that your body can learn to remain healthy in most cases.
So here you get an idea of the difference, it's not black & white and more a matter of aesthetics as both traditions do work. All the three answers following are meant seriously, hopefully they bring some clarity. I should note that I'm far less familiar with theravada than with yoga, so anybody, feel free to correct or comment the following.
1) Theravada is faster! I recommend MCTB to anyone who looks like a chronic dark night yogi, Shinzen Young to many who are new to meditation and his focus on rest even to people who are not into meditation. I think it fits with a top-heavy character.
2) Clearly mu. It really depends on the sincerity of yourself. If you're serious about it, if your attitude is good, that'll make more of a difference than the tradition you're using. [Bruno wrote that yoga alone would not have gotten him stream entry that quickly... Bhakti and the will to surrender can transform you as fast as you can handle it, that's my experience; but then, most heavy duty yoga practitioners would not line their experiences up with the insight maps, so it's hard to disprove.]
3) Yoga is stronger in the long run! Because it teaches you how to develop the good attitude mentioned under 2) that I consider essential.
The rest of this post I'll explain point three. I'll use "yoga" as it was used more than 200 years ago, so today you could call it karma, jnana, hatha, bhakti or raja yoga. I'll start with its roots:
Yoga is based on samkhya philosophy. The essence is this, you have four constituents of the universe, purusha and three gunas / threads that weave the web of existence. Purusha is the seer unfortunately identified with the seen. The gunas are the tendencies found in matter, not material substances, and every thing has all three of them. Their proportion differentiates thins.
purusha is existing, knows, and is ever happy;
sattva is intelligence and the will to grow, it can be beautiful or enamorating;
rajas is force, attraction and repulsion, it can be nurturing or desctructive;
tamas is form, it can be a suppportive structure or the resistance to change.
In Yoga you let go out of strength, you develop all (or the most important ones, a matter of choice) of your human layers to such a sattvic quality that nothing (here: tamas and rajas) disturbes the freeing of your soul, your purusha. Sattva does not cling *. But to turn a layer (a kosha, e.g. physical body, pranic body, the mental sheaths, ...) sattvic, you'll have to learn how to use it, you've to restore its harmony and learn about its purpose, powers and limitations. You master something to the degree that you can see that masterhood needs humility. This learning process is not only meant to restructure your citta, but also to change your attitude. This growth in character is described in yoga with the bhavas **.
Yogic psychology ascribes to the citta, the personality complex (basically all that is seen by purusha), 8 bhavas, 8 ways of being: dharma, jnana, vairagya, aishvarya, and its four negative counterparts. What matters for a student of yoga is not what he does, but out of which attitude. And a good attitude comes with the understanding of the gunas (karma). While you gain this understanding, your intentions more and more reflect the following four bhavas:
- Dharma, that means you do what you have to do because ... well, someone like you in this situation has to do it - the attitude of a karma yogi, a sense of duty done for its own sake, the good side of tamas (form, stability, habit, routine & behaviour);
- Jnana, which means knowldege: citta learns about itself, what serves it and what does not - obviously the attitude focused on in jnana yoga, but also in hatha yoga; it's the good that rajas has to offer, energy and nurturing and making decisions;
- Vairagya is a healthy distance so that you can see clearly, it is detachment to the point of witnessing and surrender; this is what you do primarily in bhakti yoga, at this point sattva (clarity, intelligence, harmony) predominates the other two gunas and one-pointedness is easy;
- Aishvarya is the forth, it's trust in God (Ishvar), the power of belief, spiritual strength that grows with the other three and a personal relation with the divine - raja yoga and tantra focus on this, and the purusha's qualities belong here.
According to the yogic tradition, the bhavas are naturally developed in this order, dharma supporting jnana, jnana leading to vairagya. When your attitude reflects these four bhavas, everything done with this attitude leads you toward liberation. Now if you want to strengthen your bhavas, you do yoga, be it karma, jnana, ... . If you're familiar with Patanjali's ashtanga yoga, you have two angas for the cultivation of each bhava. But every technique can be used within this scheme. Vipassana cultivates primarily jnana.
Maybe you know how Kenneth distinguishes between realization and developmental enlightenment (see Buddhist Geeks #157). Yoga is about realization, but unlike direct path traditions where you start with sitting meditation (very holistic), it starts with the development of your citta (but that's not straight developmental enlightenment). Your attitude has to change. And as soon as your citta shows signs of vairagya, you can be taught direct path teachings that lead to citta vriddhi nirodhah, the cessation of the modes of the personality complex to a state without right or wrong knowledge, memory, imagination or sleep, to quote Patanjali. If you would be taught these in a rajasic or tamsic state, the teacher would most likely have wasted his or her time.
That's just my take on this, someone doing exclusively a "specialiced" yoga (karma, jnana, hatha, bhakti) or someone without knowledge of samkhya and the three gunas (hey, they're the three characteristics of this tradition and on a par with sat-cit-ananda, but don't tell anybody) will definitely give you a different answer. Still, even if a yoga practitioner puts it in other words or never heard the word Ishvar, it's still very likely that the practices they're doing were created to master one aspect of their citta and to develop the four bhavas to such a degree that the gunas are suppporting his or her awakening. A yogi whose citta and purusha do work together in this way is described as having bhava. The yogic development is aimed at developing bhava, not enlightenment. Then strong bhavas will get the job (if not every job) done.
* compare with MCTB, seven factors, it's just strong sattva.
** see MCTB, the chapter on fractal theory.
*** all traditional yoga (karma, jnana, hatha, bhakti and raja) was developed for enlightenment only. Tantra then added bogha (having fun) to moksha (enlightenment). And I am a big fan of Aurobindo, he added yet another one: he said (integral) yoga has three aims: developmental enlightenment, non-dual realization, and divinization of matter. But that'd be off-topic here.
What you shold keep in mind is that the game is not over at fourth path. I'd speculate that you'll have to do less laundry afterwards if you follow a more holistic path that includes psychological growth in the traditional sense. But it's really a matter of what you expect a "spiritual tradition" to do with your self. When you are a yogi (= practitioner of yoga ... why do Buddhist practitioners call themselves yogi? hm, I've a guess ...), you don't need much extra. But I guess you already know Theravada or Zen practitioners who start doing ... well, yoga or tai chi or qigong as a support. But I also know two yoga teachers in my town who teach a soft hatha yoga only and meditate zen-style and sadly have not met one who learned and would teach raja or bhakti style meditation. Yoga, the real thing, can be quite self-sufficient, that's part of its beauty, but then I am not here on this forum for no reason