Hi there. Just wanted to share our recent experiences with two dive agencies.
We were open water certified with PADI. And have switched to SSI for our Advanced Open Water training. Had we been aware of the differences in the beginning, we would have liked to have skipped PADI all-together and began with SSI.
Problems we experienced with PADI (in a nutshell)...
1) After Open Water Certification, we signed up for a deep water class (not knowing it was with SSI), wishing for it to count for our PADI Advanced Open Water. After we realized it was different agencies, both our local PADI dive shop owner (Chattahoochee Scuba in Columbus, GA), and the PADI National Phone Number said that the Deep Class would not count for our certification. Their reasoning being, that it wasn't taken with their instructor. Even though, our SSI dive shop said that because of the standards of certification, transference between agencies should never be a problem.
2) We purchased gear on our own and over the internet a few times. Each time we did so, we were met with attitude from the PADI dive shop employees because we didn't purchase equipment through them. The PADI dive shop was very small, didn't have any variety, and charged more than other options we found. Regardless of those things, they wanted their cut of our purchase and were snotty about us not buying through them.
3) When we looked at the requirements for PADI Advanced Open Water instruction, they seem padded. The impression that we got was that PADI requires all these extra adventure dives, and doesn't count NITROX class towards Advanced O.W. simply to milk their students for more and more money.
4) After having taken classes with both now, we've found SSI's instruction program geared more towards actual understanding than checking blocks that students have completed something.
That's been our experience, I hope it helps. :)