Ugh, all the books are pretty useless, it all comes down to the instructor, between the open water book and the regular nitrox book, I really didnt find any additional useful info in any of the intro to tech, advanced nitrox etc books. So it really comes down to the instructor and what they teach.
I don't beleive that you can seperate the instructor from the agency.
Take me as an example. I can teach the PADI courses - and I could teach a good one. But to do so, I would have to bend many of the course standards and probably break a few. I would have to contradict the 'ethos' of the course and re-educate a student about several core principles that are presented by the course material.
Personally, I am not prepared to do that. Some PADI/DSAT instructors do. And they do a good course - but what does that tell you about the instructor? That they are prepared to break the rules that are imposed upon them by the agency they teach for. Well, what other rules are they prepared to break? Are they behaving "ethically" by breaking the standards in order to provide a good course?
As much as an instructor has to have faith in the agency, the agency has to have faith in the instructor corps. As the OP has specifically asked about PADI and TDI, I will comment on those first.
TDI standards are simply a list of things that the student needs to be able to do at the end of their course. It is a set of minimum skills. TDI trusts me to put those skills together in a sequence that suits, they trust me to add in the extra skills that I see fit to ensure that my student can do the dives the course qualifies them to do.
PADI explicitly tell me what skills I must do, in what order. This prescription doesn't take into account anything with my logistical challenges. PADI simply don't trust their instructors to do a good job, which makes me doubt whether the majority of PADI instructors can teach effectively if they are not trustable.
So agency and instructor or inexorably linked.