The 9, with the right seat will keep you comfy enough for long rides. Your feet will e out in front of you. I really like mine.
Both my wife and I have had back surgery, and we love to ride. I will also add that neither of us has gotten any relief from our surgeries either. There is actually a real term for it, (FBSS) Failed Back Surgery Syndrome. Hope you get some relief!
My 900 had a mustang seat on it when I bought it, decent but still not soft enough. Upgraded to the Ultimate seat and we both love it. It did push me forward a bit compared to the mustang. But an easy adjustment on the hwy pegs was all that was needed there. That coupled with the sissy bar pad for her it's been great. I don't prefer a backrest for me at this time, sits me up too straight. Which is good for posture of course, just not for me. The Ultimate seats are very expensive, but to date it's the best mod I have done coupled with the 8" mini apes anyways. The bars have eliminated my neck pain as well. The oem beach bars had me leaning forward a bit and always had to hold my head up. Pain got real bad when wearing helmet. That Russell seat in previous post looks real close to the Ultimate seat for the driver, but a bit small for the passenger I think. More room for the wife on mine I believe. The Ultimate sissy bar pad was much better than the oem or the mustang one I did have as well. They compliment each other perfectly with comfort and dimensions. We also still stop at least every hour for a quick break, but at our age honestly that would probably be happening even without the bad backs. Good luck!