As an instructor with both certifying agencies I can tell you that the biggest difference is brand recognition (coke vs pepsi). Both agencies are members of the RSTC. The recreational scuba training council. As such both organizations teach to the same standards and completely recognize all levels of certification up to the instructor level between the 2 agencies. There are more than 15 major certifying agencies in the world and as a rule, all registered dive businesses will take you fun diving as long as you have a certification from any recognized agency. PADI and SSI completely recognize each other to the point where you can complete the SSI open water and then the PADI advanced if an SSI facility is not available at the time of furthering your dive education. Naturally it works the same way with PADI to SSI. To me the greatest benefit of SSI is that it offers the online training for free where as PADI charges up tp $120 for their online system.