The single most important piece of information you need is whether or not the certification organization is NCCA accredited.
After making sure of that, the rest is up to you. Anyone can pass a test if they put in a little effort. Experience is the next hurdle.
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Once you are certified, you need to keep up youreducation and get your CECs for renewal. Think about coming to the Big Island for a Continuing Educaton Vacation and you should be able to write off most of your expenses (if you set up your week/weeks with me properly) for the trip.
Ask your tax guy. According to my tax advisor, “Professionals must spend over half of their days doing business (in a CEC course for some part of the day) to deduct airfare. Other costs (Hotels, meals, rental car) are only written off for the business days (days of CEC courses).”
I set up CEC courses for individuals or groups according to this, their budget, and how they want to set up their time. I can take you around the island for off site course work and out on field trips like swimming the triathlon course, biking, hiking the volcano,and more with CEC time, etc. And you get to come to Hawaii!!
I want to commend you for contemplating a group exercise certification even though INSANITY only strongly suggests but does not have it as a pre-requisite.
I would go with an widely recognized certification such as ACE as a base certification. After all, you may feel at some point in time that you want to branch out beyond INSANITY, and then you can still teach a variety of formats.
Here is a tool that will let you compare some of the major certifications. It is created by ACE (that offers its own certification), so of course good to look at other places as well. The top lists usually include ACSM, ACE, AFAA, and a few others.
You might also look in your local area to see what certifications other instructors have, and which are accepted by most of the places where you would like to work.