I teach for PADI and SSI (and BSAC).
The differences for you as a customer are:
1. SSI don't make you buy a manual...you can loan one from the shop (if they are willing) and that can cut costs.
2. SSI teach primary donation (as well as octopus donation) for OOA and they teach Buoyant Emergency Ascent in confined water (as well as CESA).
3. SSI certification applications cost less than PADI Pic cards...saving you money.
4. SSI courses allow more than one focus per dive. i.e. You can be credited for doing Fish-ID whilst on a Deep Dive....or Buoyancy skills whilst on a Dive Computer dive. PADI do not allow this.
5. PADI have a checkable internet database of its divers...great if you lose or forget your c-card.
6. PADI dominates the global diving market (although in Oz it is 50/50 for PADI and SSI - in Thailand it is 75/25, PADI to SSI......much less than that for SSI in other regions), so the certifications are more easily recognised (never seen a problem with recognition of SSI certs anywhere though) and verified by the internet database mentioned before.
7. Both PADI and SSI are RSTC (Recreational Scuba Training Council) members and, so, their courses are virtually identical.
The differences for you as a Potential Professional Diver are:
1. The SSI Divecon course equates directly to the PADI Assistant Instructor course - there is no 'DM' course with SSI. So, you get two courses for one.
2. PADI is the market leader, so employment potential is better as a PADI Divemaster. Many SSI schools are also PADI schools...so you can get work as an SSI Divecon, but you could also have got that work as a PADI DM. Basically, PADI DM is a lot more employable.
3. If you want to become an instructor later on, you will want to choose PADI...it is the biggest employer and the only agency which doesn't allow direct cross-overs at instructor level (SSI will allow PADI cross-overs for a cheap/quick course). Doing a PADI DM course gives you many advantages if you later choose to do a PADI IDC.
4. SSI are more flexible about how their instructors teach courses. Most SSI instructors prefer that flexibility over the rigid PADI system.
5. PADI has a global employment board (for what its worth), wheras SSI have regional boards (that tend to be a little more effective for job seekers IMHO).
6. SSI Dive Shops have control over their instructors (they even write annual reports on them for QA purposes)..and SSI instructors have to work for an SSI 'dealer' (dive centre). With PADI you are an independant instructor, meaning that you can teach where you want - you can teach who you want (courses for family and friends etc).
For me...I like being an SSI instructor...but only because I am already a PADI instructor - so I have the option when looking for work. I'd always have chosen the PADI IDC to become an instructor and the DM course was an excellent preparation for that.