Don,
My experience with PADI cavern vs the others with regards to continuing ed to intro and beyond isn't so much that PADI is viewed as being less, as it is that the instructor may be viewed as being less.....or simply the TURF wars that are rampant in cave country....both Florida and Mexico. Each agency knocks the other, that's a fact. Pick your instructor, not the agency.
FWIW, my buddy and I, both active cave divers taught PADI Cavern to a family of 3 who didn't initially want anything beyond that. When they decided they were interested in trying Intro, we recommended a few NSS instructors for them to consider. Once they made their choice, we personally contacted the instructor and gave said instructor an overview of what was covered, how they performed, etc.
The class went well and the NSS instructor informed us that we had done a good job AND did not make them start from the beginning after doing a shakedown dive with them.