I know the standards. An instructor from any other agency has to complete IDC and IE. However, they may skip the AI portion of the IDC.
When I later crossed-over to SSI, all I had to complete was a 1.5 day orientation to get the instructor rating. This would not have been possible the other way around (into PADI).
The PADI IDC and IE is expensive. A short cross-over is cheap. If you have to do ONE full instructor course.... my point is that it should be with the agency that does not allow direct cross-overs.
And you contradict yourself with this. PADI insist on a full training program. Other agencies accept simply cross-over of qualifications. That was my point in the first place.
Well, in my experience (teaching for 3 different agencies), there is very little that varies when it comes to teaching someone how to dive. But, as a professional instructor, the fact that PADI dominates something like 90% of the global, recreational, diving market...seems to provide a good reason to choose them as a certifying agency. From there, it is a simple, and cheaper, process to convert to another agency in the future, should you ever desire it.