Getting back to the topic of the original post....

While it's true that the basic standards for agencies that adhere to the RSTC guidelines are pretty similar, there are differences. As noted by BK Diver, some agencies encourage instructors to go beyond the curriculum while others prohibit it. There are pros and cons to both approaches, but that is really a topic for a different thread, so I'll leave it there. The main thing is to make sure *you* are comfortable with your instructor and neither intimidated in any way or uncertain of his/her capacity to do a good job with you (you are allowed bias here, even if it's not very PC--you may prefer a male/female/teenage/middle-aged/senior instructor with a boot-camp/nurturing approach or anything in between!).

In addition, there are differences in the degree to which agencies embrace technology. Both PADI and SDI now have e-learning options that allow students to do most of the academic work online before meeting an instructor face-to-face for the practical in-water sessions. SDI was actually the leader in this approach, and PADI very recently jumped on what seems likely to become a bandwagon. SDI also *requires* students to train using a dive computer to plan dives whereas PADI still requires students to learn to use tables for dive planning. I expect dive tables will eventually become redundant (though a lot of us like redundancy when it's our lives in the balance), but it may take a while due to the economics of dive centers needing to supply the equipment. Already, most tech divers do all of their planning using computer software rather than using the cumbersome tables to work out those complex dive plans, so why not recreational diving. (After all, what else is a dive computer but dedicated software in a tiny bit of hardware?)

Hope that helps!