My goals are to be technically proficient and rise to advanced open water, at a minimum. So, you'd recommend SSI over other agencies? How do they differ from PADI and others?
Forget about agency. Every agency has great instructors and not so great. Some instructors are people I wouldn't get in the pool with and feel safe knowing what I do now.
Select your training based on the instructor and what additional materials they add to the course. A by the book, taught to standards course is often enough to get you to where you can survive.
Choosing an instructor is actually hiring an employee to work for you. Their job is to make you proficient, skilled, safe, knowledgeable, confident, and comfortable in the environment.
You should select them based on an interview you would conduct as if you were hiring any other person to do any other job.
I have a set of interview questions I suggest people use when selecting an instructor. The full version of them are in my books but for space considerations here they are without the answers I'd expect to get and why.
1. When did you become an instructor?
2. Have you had any large gaps in your teaching career? How recently? How many courses have you taught in the past year?
3. What agency do you certify through?
4. How much time is required for the course?
5. How large are the classes?
6. What equipment do you provide and what do I need to provide?
7. Is the schedule set or flexible?
8. Do you teach skin diving skills?
9. What methods do you use to teach proper weighting?
10. What methods do you use to address the panic cycle?
11. Do you teach students to perform skills in a horizontal position from the beginning of the class?
12. Will I have time to just swim and practice the skills we learn?
13. Can I sit in on a class or pool session?
14. What methods do you use to teach buoyancy and trim?
15. How much time will I get to practice these skills?
16. Do you dive with students after the class has ended?
17. Where do you do check out dives?
18. How many places have you been diving?
19. When was the last time you were diving?
20. How often do you dive for fun?
21. Do you have references - former students that I can talk to?
These are a start. In some instances you may want to find out if you'll have the same instructor for all phases of the training.
No good instructor or operation should have any trouble with you asking these things and any others you could come up with yourself before you sign anything or hand over a nickel for training.