When I did my OW cert 8 or so years ago I had heard of PADI and NAUI but no one else. I did mine through PADI because the shop at the resort I stayed at was PADI, honestly for no other reason. When we started looking for a shop for the wife to do her OW, it was still very much related to location. The first shop we went to was SSI, and we began talking with the owner and one of the other instructors who would be doing her instruction and hit it off with them, so she's SSI (as is my skills update). Her OW was well done, I thought. She had some issues when she first got in the pool with nerves and her mask filling with water. Instead of simply stamping her card and calling her good to go, the instructor pulled her out of the rest of the class, worked with her one on one (including a pool gear check where it was discovered that her mask issue was because her original high volume mask simply didn't fit, so they traded it out with a low volume one that was watertight [and refunded us the few $ difference]) for a couple of nights a week for a few weeks to make sure she truly was comfortable in the water and with the skills-all without extra cost to us for the additional sessions. As dwatts noted, from our perspective right now (newbies that are really *maybe* a few steps above 'vacation/cruise' divers), it's almost entirely about the instructor, and not the cert agency. Now, we are planning to get into some higher complexity down the road, and I would imagine that the differences in the agencies will show themselves as we progress, but, bottom line right now, it's about our comfort with the instructor, not the oversight body.