I used to row with a high back, I felt it gave me more back support and leverage to lean into. I have a lot of back pain/chronic injury problems. Early last year, I tried a low-back, and was practically blown away by how much better my back felt by being able to move around, and I could throw my weight around more since the back of the seat wasn't preventing me from leaning further back. Now I use low back on both my raft and cat, and love them. I also wear a high float pfd and did not like how it bumped into the high back, getting in the way even more, and on the low back the pfd clears it.
I'm amazed now watching really good catboaters with high back seats, it just goes to show you there are so many different ways to rig, and the best solution is what is best for you for your specific needs and issues.
The only downside to the low back is that I don't have a nice back to lean on, which I think has actually been good for me as I'm forced to maintain a good spine position on my own, which ends up strengthening my back in ways I need.
Key note, I use the old style low back, the newer gray NRS ones I do not like AT ALL.
What kind of frame are you going to have, and where are you going to sit? Sitting on a drybox all the time is going to be quite a bit of extra weight that you won't need on day run (and crap to rig), and also hang down low and might get in the way of straddling rocks or other stuff. I don't recommend doing that as a first choice, I'd say go with the cross bar. I put a drybox behind me on multi-days, and that works well. You give up some storage space not sitting on a container, but it depends on the primary purpose of the boat, and for me and my lengend it's day trips, and if you're talking about a small legend it's not a gear hauler anyways. I use the open compartments next to the seat cross bars as storage bays also, so they are useful on day trips, and a lot of people put various containers there.
Living in Montana, boating in Idaho