But what's the difference?.
There is a lot of difference,and this can be something to pinpoint.
The older agencies are well established,and recognized,some of the newer agencies can have difficulty being acknowledged after training. For example,there are cases of cards not being accepted universally worldwide. Diepolder has written they only accept CDS and NACD cards,and this may have changed. I know of a person presenting a card at another site for an agency that was new,and they got rejected-because there are fake cards and invented agencies out there,thanks to desk top publishing.
My personal issue is how a particular agency makes their instructors. This may be one reason you may see quality instruction with ABC,but not XYZ. Some agencies require internship and peer review at an institute,others require a weekend with a course director. I am not saying the later is better than the former,but you have to decide.
I think the GUE model mentioned here is good,especially if you are a new diver,because their fundamentals class will make sure you are well prepared before advancing. All too often,people will show up with new gear they have never used,with the attitude of make me a cave diver-first we got to make you a diver. The only issue I have with GUE is their rigidity to a system that has minimal flexibility within the system.
When I started cave diving 20 years ago it was very simple CDS or NACD,but now it is very confusing,with even some marketing thrown in there to persuade to student to choose their route because we will let you do more at the basic levels. If I had to do it all over with what I know now,I would consider the fundmentals class because it would solve a lot of problems I had in the beginning. Then I would pick an agency for cave training that was well established with a long history,and find an instructor that was very active,and had been doing it for many years.