I have had the good fortune to train a little bit with Mike Sigman (at one of his Seminars and a couple of days of personal instruction), Sam Chin (at his Seminar and with his father Grandmaster Chin in Malaysia), Seichiro Endo (at various Seminars and he will be giving a Seminar with us in HK next year), as well as a number of Yiquan and Baguazhang teachers in Hong Kong in addition to my primary teacher. Some of these experiences are detailed on my Blog
I have not had the opportunity to train with Dan Harden nor with Ark.
A few caveats:
(1) You need a bit of exposure to IS/IP to be able to feel and understand the differences (everyone is going to throw you around and you will think they are all powerful). Ignore the guys who say he touched me and I fell down like magic. It is a matter of training your sensitivity, even if you can't do what the master is doing, you should be able to feel that he is doing something to you, and then eventually piece together what happened.
(2) Your definition of what constitutes IS/IP also matters, I have tried to be as agnostic as I can in the following description
(3) All the above are effective when trained to high level, but some methods de-emphasize or emphasize certain aspects, so the question who is better in a fight is not really relevant as all these people can still kick my ass.
Here is my personal opinion on the strengths of each
Mike Sigman - has a comprehensive theoretical model based on Jin,Qi and Suit which has impressive explanatory power. Most of his model is based of off Taiji (mainly Chen taiji) and Chinese internal martial arts classical theory. His exercises will help train the body structure and development of the dantian / tanden, but necessarily take a long period of time to show results. Also he does not teach applications - he is trying to show how to generate a certain kind of power through the use of the dantian - and this "engine" can then be retrofitted into whichever car / style you are driving
I Liq Chuan - a "family" style with strong Yiquan and Taiji influence (and an overlaying of Buddhist meditation techniques). Has a detailed system, and also a systematic method of training which will allow you to control your opponent in spinning hands (his version of push hands) or sparring and developing a good internal sense of balance and control. Good results in a short time. Less emphasis on training "structure" (although he does have some standing post exercises) and little mention of dantian power.
Seishiro Endo - a soft form of Aikido with and emphasis on relaxation and "connection" - the emphasis is on controlling your opponent through forging a connection with the opponent and also executing the technique with the least resistance and effort. Good training for sensitivity and "reading your opponent" to feel where the "holes" are and also to train relaxation of the body and how to use you strength in the most ergonomically efficient way. No emphasis on training the body through solo exercises (although there are glimpses on some of his DVDs and in personal conversation with him that he might actually do this himself) and issuing power or fajin and use of tanden is completely absent.
Yiquan - good training for building a strong body structure that is relaxed but does not collapse and can transmit power efficiently. Good for health and an indeed at higher levels for building a blow-resistant body. Indeed many of its standing post exercises have been co-opted by other arts such as Taiji, I Liq Chuan, etc. However the quality of the instruction is hugely variable, many teachers do not teach applications or know how to fight, and there is some controversy as to whether it uses the dantian.
For baguazhang and Taiji the quality of instruction ranges so widely that its not meaningful to comment.
Just my two cents. Hope this helps.