People often ask me to compare Tai Chi vs. Yi Quan. Not that I'm the guy to ask. There are much better experts out there, find one of the big master teachers, why don't you, and ask them. I'm a nobody in this field.
Yet people do ask me. So this post is my final answer.
First, from the high concept level, let me emphasize: They are basically the same thing. They have the same goal, which is complete permeation of the body with internal energy that is instantly moblizable for combative application. There's your vision statement thing, right there. So don't get on my back about high level stuff.
Now, from the implementation point of view, there are differences. But again let me caveat that these differences in the arts will often or largely be washed out by the diffferences in individual practitioners. People like different stuff, people have different needs, goals, preferences, and prejudices... all that kind of thing will largely wash out any theoretical advantage that one set of practices may appear to have over another. So again, stay off my back about it.
So, what is it? Well, they really are similar. Problem is that there are many sub-varieties of each art. In some cases, the differences between two sub-types of the same art can be greater than the differences between that art and a different art. So I'll narrow it down to comparing ZMQ37 Tai Chi to the Yao Brother's version of Yiquan. I learned that in a total of 4 months of 7 days / 7 hours-per-day work at the ZongXun WuGuan in Beijing under Yao Chengguang, arguably in some sense the current generation leader of YiQuan overall (yeah I know you are gonna get all in my shit about that statement, but guess what I don't care much one way or the other. It's merely a point of orientation for this post).
They really are very similar. Both are primarily intended to develop the internal energy for combative application. Both use a reasonably small set of postures and movements for that purpose. Both emphasize holding postures for longer or shorter durations. Both have a set of partner drills called push hands (tui shou). Both claim to emphasize mind training and physical relaxation as the basis of the work.
In practice though, really down on the ground, or down in the dirt, there are very important differences. Main thing is the degree of relaxation. When you learn the Yi Quan static holding or shi li (slight movement) holding, Yi Quan coaches will indeed emphasize relaxation very much. So you wouldn't think that would be an issue. But it is. Simply, the structure of the Yi Quan postures leads to more unconcious tension, especially in the upper body, than with the ZMQ37 equivalent work. I saw so many examples of this that I regard it as inarguable. But check it out for yourself, don't believe me. Since it is mostly unconcious tension that is engendered, if you ask any YQ practitioner about, they will strenuously (tensely?) deny the point and forcefully argue how relaxed they are. But then do some push hands with them - you'll detect it instantly. Like touching a wall.
In fact, their entire approach to push hands actually explicitly relies on giving tension to your partner to work with. I do understand their reasonong for this, so I won't go deeply into it here, but the fact remains that this is tension, it leads to long term unconcious tension, which will no matter what anybody says hinder the internal development.
Now you may say: well ok they are more tense but you NEED that tension for "real world" applications and sparring. This isn't a Tai Chi girly-man tea party, this is real he-men doing real shit like sparring ... oooooh! Shiver~! Fuck it. That's not a very good answer in terms of this post because I am comparing the arts in their long term effects and results here. Yes, it's true that if you match up, all else equal, a one-year ZMQ guy against a 1-year Yi Quan guy then sure - the YQ guy will quickly punch out the ZMQ guy, and lights out.
But that's not a final answer, because if you really want to learn how to hit somebody fast, hard, and accurate, in less than one year just do 6 months of Western Boxing (including sparring) and you'll punch out the equivalent YQ guy easy. So that kind of comparison is meaningless.
As for training rigor, it's odd that while most people are very impressed when I say that in Beijing I used to stand every morning for one hour in "combat post" right side, then immediately switch for another hour of "combat post" left side, fact is that wasn't so hard. Strangely, it is far more rigorous and difficult to hold the ZMQ postures for 10 minutes than to hold YQ for an hour. And also more internally productive.
Another thing is the very much sportive atmosphere of YQ, at least as done at ZXWG in Beijing. They have a tense, competitive mindset. Of course that is cloaked by all kinds of "health" talk and so on, but basically they have a gamey, competitive, young man's mindset. Which is good in a way, I respect practical athletics. But again, boxing is the same thing but more honestly presented. This kind of mindset just adds yet more unconcious tension to the average student.
Long term, in my opinion, despite the considerable similarities in goals and even methods, the ZMQ method due to its much superior technical design in support of relaxation training and elimination of unconcious tension (and methods for detecting and exploiting unconscious tension in others), has it all over the YQ. Not to say there are not great fighters and masters coming out of YQ of course there are. This is just my peronsal stupid dumbass opinion: if you want to aim for the highest levels, ZMQ is superior.
Don't quibble with me on this writeup. If you don't like the way I've done it, do it yourself.