Both my local shops have been PADI, SDI, and TDI shops since before I started diving 1 year ago. Both shops added SSI this year.
One of those shops was a PADI 5 star dive center in good standing, with membership paid up through the end of this year. When the PADI rep found out that they were adding SSI, the shop's PADI 5 star rating was immediately dropped to a 4 star. Then the shop was mysteriously deleted out of PADI's online database (i.e. where a prospective student goes to PADI's website and searches for a nearby shop). The shop complained and they were added back. I think they mysteriously disappeared another one or two times. Now, they are so pissed at PADI that they are not going to renew with PADI, no matter what, and will be only SSI, SDI, and TDI.
That same shop also told me some of the reasons why they liked SSI enough to get crossover training for all their instructors and make SSI their main offering (prior to all the pissiness with PADI).
SSI pushes prospective students to the local shops. In contrast, PADI pulls students to their website. Example: With PADI, anyone can go online, pay PADI, and do a full e-Learning course (say for OW Scuba Diver) before they ever talk to anyone at a shop. With SSI, anyone can go online and do the first 3 chapters of the OW course for free. But, to continue and finish the course, they have to go to an SSI shop and pay the shop for the course, at which time the shop unlocks the rest of the online course for the student. With PADI, the student pays PADI before they start anything. With SSI, the student never pays SSI anything. Anything they pay for, they are paying the shop they are working with. This has had a side effect over the last 6 months that the shop's students' rate of buying full gear has gone from single digits to something like 70 or 80%. That's not just good for the shop. They feel like students who buy a full set of their own gear are much more likely to continue diving, so the shop feels like it's actually good for the students and the sport also.
PADI rigorously defines what the student is taught and how they are taught. The aforementioned local shop disagrees with some of the details of the way PADI requires things to be done. With SSI, they have standards too, but they leave a little bit more up to the judgment of the instructor as far as HOW the things are taught.
I could be totally wrong about this, but I *think* I remember the shop told me another difference is that if there is a class in the water with an instructor and an assistant, and there is a student who is having an especially difficult time with a skill, under PADI the approach would be to have the assistant take the problem student aside to work with while the instructor continues with the rest of the class. Under SSI, the instructor would work with the problem student and the assistant would continue to work with the rest of the class. If I am recalling this correctly, that was another reason the local shop liked SSI better as having the full instructor work with a problem student seems to make more sense than having a non-full instructor do it.