Way back when I was trying to decide between the two it was explained to me that the basic difference between SSI and PADI was two factors; PADI is more universally repected. And SSI lets you train with dive computers and skip the whole nitrogen tables portion you need to learn with PADI. I keep my PADI tables in my dive bag as a backup, but to be honest I have not used them once since I became OW certified and bought my own computer. I may be totally wrong and off base with this comparison, but it’s what I remember.