Yes, I'm a PADI instructor, Marci.
Here's the big difference. PADI, as well as some other agencies allow their instructors to self-certify. What this means is after getting a certain certification and doing 20 dives post certification, as a PADI instructor I can apply for that particular instructor rating. PADI used to only require its cavern instructors be Intro to Cave certified with 20 dives post cert. At that point they sent in an application for cavern instructor as well as a credit card number and a couple weeks later a new instructor card came in the mail. PADI has changed the standards so its cavern instructors have to at least be full cave now, but they can still self-certify. There are still Intro to Cave PADI cavern instructors since they were grandfathered in when PADI changed the standards.
NACD and NSS-CDS both require long internships and instructor examinations before an instructor can become a cavern or cave instructor. The internship involves working with 3 different instructors through each level (cavern, intro to cave, apprentice, and full) and then going through the 2 day instructor examination that is staffed by 2 additional instructors. So by the time the process is done the new cavern instructor has been evaluated by 5 different experienced instructors.
It's rare that I will accept a PADI cavern diver into an intro to cave class because their skills aren't up to the standards I and most other cave instructors who teach for the cave agencies expect. This means if you take a PADI cavern course from a PADI only instructor you will likely have to repeat the cavern course if you decide to move beyond that level. I will accept a PADI card from other instructors that I know are also NACD or NSS-CDS instructors because we all teach the same course. So at that point it doesn't matter what card you get. It is about the instructor. Just make sure the instructor does not only teach for a recreational agency.
Here's the big difference. PADI, as well as some other agencies allow their instructors to self-certify. What this means is after getting a certain certification and doing 20 dives post certification, as a PADI instructor I can apply for that particular instructor rating. PADI used to only require its cavern instructors be Intro to Cave certified with 20 dives post cert. At that point they sent in an application for cavern instructor as well as a credit card number and a couple weeks later a new instructor card came in the mail. PADI has changed the standards so its cavern instructors have to at least be full cave now, but they can still self-certify. There are still Intro to Cave PADI cavern instructors since they were grandfathered in when PADI changed the standards.
NACD and NSS-CDS both require long internships and instructor examinations before an instructor can become a cavern or cave instructor. The internship involves working with 3 different instructors through each level (cavern, intro to cave, apprentice, and full) and then going through the 2 day instructor examination that is staffed by 2 additional instructors. So by the time the process is done the new cavern instructor has been evaluated by 5 different experienced instructors.
It's rare that I will accept a PADI cavern diver into an intro to cave class because their skills aren't up to the standards I and most other cave instructors who teach for the cave agencies expect. This means if you take a PADI cavern course from a PADI only instructor you will likely have to repeat the cavern course if you decide to move beyond that level. I will accept a PADI card from other instructors that I know are also NACD or NSS-CDS instructors because we all teach the same course. So at that point it doesn't matter what card you get. It is about the instructor. Just make sure the instructor does not only teach for a recreational agency.