>>2760889At entry level i'm tempted to say PADI is marginally more complete than SSI. When you go into specialisation SSI has better offers in my opinion. I strongly advise going for either of these for your "core" certifications because every dive club in the world will be able to connect to their databse and confirm your are a trained diver and can dive there. That may not happen with "small" certification entities.
Depending where you live you may have a solid national-level organisation. Where i live it was created decades ago by ex-combat divers and still keeps a very "extreme" culture, it offers excellent training, internationaly recognized, but the lessons are dry as all fuck. First thing PADI does is dropping you in the water at 3 meters so that you can get a feel of what's going on, then you have the theory that explains what went on to you. My national federation has something like ten hours of theory before you touch water.