I thought most folks on this thread agreed that the BASE curriculum is similar for PADI and SSI. No surprise that the books are similar. And if all you expect to be taught is the base curriculum, then you are correct the instructor is the sole variable.
Only when you have the choice to be taught MORE than the BASE curriculum will the agency come into play. One will encourage it, one will not. When a wonderful instructor must withhold certain skills until you have signed up for the appropriate specialty course, ask yourself if it was him (the instructor) or the agency.
I obviously can't say if SSI or PADI has a policy of withholding information as you imply, I'm not an instructor for either. I'm curious to see which agency you feel encourages instructors to withhold skills. I didn't have that experience with my SSI instructor or my PADI instructors.