THE BOTTOM LINE...
... is, they are both very similar. SSI theory is online only and when it comes to your course, more flexible in the course delivery. PADI has paper manuals for all courses and online for some courses, but is a bit more rigid with course delivery.
Poseidon recommends SSI for three reasons;
Continue reading for a more detailed explanation.
Both agencies have set standards based on years of research and industry best practice, both follow guidelines set forth by the WRSTC (or World Recreational Scuba Training Council) and both have certain courses (Open water for example) that meet or exceed ISO standards. As a student, you would hardly notice any difference between the two but as an instructor, there are a few differences. Standards govern things, such as class sizes and depth limits based on age and experience. They also determine what skills are taught and the sequence of those skills. Many instructors prefer the SSI approach, which gives instructors much more flexibility in the sequence of skills that the student learns, while PADI maintains that skills cannot be moved from one in-water session to another, although the order of skills can be altered within a session.
The PADI philosophy is for students to complete the Open Water course, go on and do the Advanced Open Water course, supplement their diving education with Specialty courses, and ideally, take the Rescue Diver course. A recreational diver can reach the level of Master Scuba Diver by completing the Rescue Diver course and 5 Specialty courses, as well as being able to show evidence of having done 50 dives.
The SSI path is slightly different. The focus after the Open Water course is more on Specialty courses to enhance and broaden the divers skills. SSI do offer an Advanced Adventurer course, which is very similar to the PADI Advanced Open Water course (5 dives, each dive is the first dive of a Specialty course) and if you chose deep as one of the dives it also allows you to dive to 30m, the same as the PADI Advanced Open Water course. But to be recognised as an Advanced Open Water diver with SSI, you need to complete 4 Specialty courses and have 24 logged dives. SSI will automatically award this rating on meeting these requirements, with no additional charge. SSI has other awards that recognise training and experience, that are automatically given at no extra charge. These include: Master Diver (4 Specialties + Diver Stress and Rescue + 50 dives), Century Diver (100 logged dives), Silver 300 Diver (300 logged dives), Gold 500 Diver (500 logged dives).
PADI still offers some course materials only in printed form, but many of the common courses are offered as eLearning. We recommend the eLearning route so that you can complete your study at home before your holiday in Cyprus. This leaves you free to relax in the evenings instead of worrying about homework. Of course, it is always possible to purchase your printed manual in your home country and study before you travel. SSI offers all course theory online only. The advantages of this approach are many: the material is always up to date, there is no heavy manual to carry around, and the environmental impact is lower.
Some courses are extremely similar, but some, for example the Wreck Specialty course, are quite different between the agencies. The PADI course consists of 4 dives, which involve noting hazards and points of interest, navigating on a wreck, laying and following a penetration line on the outside of a wreck, and either conducting a wreck penetration under close supervision or organising and executing a non-penetration dive. On the other hand the SSI course is only 2 dives, covering hazards and points of interest, navigating a wreck and using a tether line reel. Penetration diving requires the Extended Range Wreck Diving course which is another two dives so all in all it ends up being the same in the end but with only two dives you achieve the wreck specialty with the option of upgrading at any time in the future.
The good news is that both SSI and PADI are globally recognised training agencies, so no matter where you dive your training and qualifications will be recognised. This means you are free to pick and choose the course and training method you prefer every step of the way. For example, you could do the PADI Open Water course, the SSI Nitrox course, the PADI Wreck course and the SSI Deep course. This would enable you to dive using nitrox, dive up to 40 metres deep or penetrate wrecks to the limits laid out in your training.