I lean toward the Sequoia, for reasons that may not be the same as yours. I have a friend who's ridden one for a couple of years, and she loves it. If I have the models straight, it gives you a little more "roadability" than the Sirrus, which is more of a hybrid. Of course if you WANT more capability on dirt or whatever, then the Sirrus would work for you, but most people who get into cycling seem to want to go farther and faster, and I think the Sequoia will do that more easily. My Rivendell Atlantis is similar in concept to the Sequoia, and it works well (with no more than a tire change) for everything from centuries to touring and fire-roading.
Regarding your back: RAISE THEM BARS! Whatever you have to do (new stem, larger frame etc) to get the handlebars about level with the seat (certainly not several inches lower) will probably help.