Hi there,
As some might know by now I'm a novice diver who's busy getting my AOWC from SSI, the specialties I do are Deep Diving, Underwater Navigation, Peak Buoyancy Control and Nitrox. Besides the courses I've been diving every weekend since I got my Padi Open Water. I like diving but I like learning to and having the feeling of developing and improving my skills so I'm already trying to figure out what learning path I'm willing to take.
Some nice specialties: Drift, Night, Search and Recovery, Wreck, Cave and some others I like a little bit less than the one said: Boat, Digital UW Imaging, Self Reliant Diver.
My opinions:
Don't worry about the agency at all. Take the courses that offer something you want to learn and worry about whether the instructor you'll be training with is good and has a personality/teaching style that works for you.
I would not bother taking Drift, Boat, or Night. If you want to learn about those, go book yourself on some dive charters for those types of diving. Tell them you haven't done those things before and make sure they'll have a DM in the water with you (pay extra, if you need to).
I think Search & Recovery, Rescue, Wreck, Cavern, and (eventually - after you have at least 100 dives) Solo/Self Reliant are all courses that would be good to take and, of course, from a good instructor.
Solo is the one course where I would personally be specific about the agency. I don't know what SSI offers for this or if they do. But, I would personally specifically choose the SDI Solo Diver course over the PADI Self Reliant course. The reason is that I know my local quarry allows solo diving for people with SDI Solo, but they do not allow it for people with PADI Self Reliant. I'm not sure why. The shop that controls the quarry is a PADI, SDI, and SSI shop, so they don't have a reason to shun PADI. I have not heard of any place that allows solo for PADI but not for SDI, so I'm not sure if there is any downside for choosing SDI Solo, but there is at least one dive site that gives a downside to PADI Self Reliant...
If you want to really improve your basic dive skills, also consider TDI Intro to Tech, GUE Fundamentals, or PADI Tech 40. Presuming a good instructor, any one of those should yield significant improvements in your basic dive skills.
If you want to learn about U/W Photography, I would suggest doing two things: One, take one of the aforementioned courses (Intro, Fundies, or similar) to bring your basic dive skills up to a higher level. Until you take a course that challenges your buoyancy and shows you your limitations (however good they may already be), you won't really know what your limitations are and it's VERY easy to THINK your skills are better than they really are. That course should also teach you additional finning techniques (e.g. frog kick, helicopter turns, back kicks, etc. that you may or may not already be proficient with). Really good buoyancy, trim, and technical finning techniques are HIGHLY advantageous for U/W photography. And, two, purchase Martin Edge's book, The Underwater Photographer, and read it. After you do those two things, you will probably be ready to go work on u/w photography on your own. Once you get some experience doing it on your own, THEN you would probably get the most benefit from finding a good u/w photography instructor and arranging some training from them.