I have SS and I do like it. From what I understand, SIS is not as advanced and is considered to be the predecessor to SS for people who wish to progress to the level of SS.
There are issues with form and sometimes Debbie does not ( okay, more than sometimes) follow the beat of the music, but I keep reaching for these when I want to lean out, and they do the trick!
I also like Jari, and Debbie is similar, but I find SS moves pretty fast for the most part,and Slim and Lean pauses between body parts.