Going to depend onthe shop you want to work with or for that matter if you even want to work thru a shop. PADI allows independant instructors while SSI does not. To be a SSI Instructor you MUST be affiliated with one of their stores, making it more difficult if you were to have a dissagreement in that store. With PADI you can go almost anywhere.
This post and Walters post are my favorites on the thread so far.
SSI and PADI differ little in content. SSI basically copied the PADI system, changed 2 things so they can't be sued for infringements on intellectual property rights and that's it. In terms of approach they're virtually identical.
What pri8 explains is important. As a PADI instructor you have a LOT more freedom than you do as an SSI instructor.
There are, however other options that I wouldn't write off if I were you. You should query Walter on what's good and what's not. he knows more about the differences between agencies than most people (and certainly more than me).
That said, in terms of getting jobs, PADI still has a larger market share than all other agencies combined and if what you're looking to do is make a go at working as an instructor, there's only really rational choice on the international scene. Locally, you might find that you have more options.
If you want to make money as a scuba instructor then
1) you must be lucky
2) you must be willing to travel extensively
3) you must have other skills besides being a dive instructor
Margins are small in the scuba market. There aren't many instructors who make more than subsistence wages and there are many who could double (or triple) their salary by taking a job shaking the oil out of the french fries at McDonalds.
R..