Walter:
Actually, there are major differences between agencies.
SSI, for example requires its instructors to be affiliated with an SSI dive shop. Some say this ensures quality control because it allows for direct supervision of instructors. Others say it undermines it because shops are usually the ones who push instructors to cut corners. If instructors need supervision, who supervises the store owner?
PADI has very few required skin diving skills and no confidence building skills. It's a bare minimum program.
NAUI has more rescue skills in its OW course than most agencies.
YMCA has lots of skin diving, rescue and confidence building skills required as part of its OW course.
You can find good and bad instructors with any agency.
Just a couple of things to add. I'll preface this with my own background. I'm PADI certified, so I'm most familiar with them, but have taken a look at some of the other agencies. The instructor is the key factor, but you also need to look at the student. What kind of diving is the student wanting to do? If all a diver is going to do is DM guided resort diving, then PADI is probably the best one. It is a bare minimum program, but that's all some people want. As stated in another post, five pool sessions is not a lot to prepare someone for open water diving; four check out dives aren't much either. I still can't believe my wife and I went on a dive by ourselves a couple of hours after finishing our check out dives. A couple of weeks later we went to do some fresh water diving, alone. I'm not so sure we were prepared enough for that at that time. And we've seen students come out of check out dives that we thought were even less prepared. It's kind of surprising that more accidents don't occur.
Back to PADI. The bulk of the learning comes in after OW cert. If these are people that want to do more than resort diving, then either PADI OW followed by AOW and Rescue or one of the other agencies. We knew we wanted to do more diving, so we finished our AOW and Rescue certs within 6 months of OW. But not everyone has the time or money to do that.
I have heard that SSI teaches a lot of theory in their OW classes. Good to know information, but not something the typical resort diver may want to know. SSI also has higher minimum qualifications to be considered Advanced and so on.
I've heard that NAUI leans more towards the technical aspect of diving, maybe not in OW, but beyond that.
I don't know anything about the YMCA program.
I don't know how much any of these other agencies prepare you to do non-guided dives immediately after the check out dives, but my guess is that, unless they are much more intense than PADI, it isn't much different. Otherwise they wouldn't be able to get many students to stick through the course. Being PADI certified, I'm in no way trying to discredit their program. I do believe in their program, otherwise I would have crossed over long ago. But you do need to have the time, money and commitment to pursue continuing education and learn some of the finer aspects of diving.
If I gave any incorrect information here, please let me know. This is all just my opinion from the limited knowledge I have about the other agencies.