I can only give you my personal experience, since I am certainly not an expert on touring bikes. I've faced a similar question recently. I have a herniated disk (injury far predating biking) which recently started to act up. I ride my bike 75-100 miles a week, mostly commuting with a few weeks of touring in the summer. My doctor and physical therapist were both concerned about road vibrations. I must admit, I could definitely feel it in my back when I hit the many cracks in the road. The roads in eastern MA are pretty poor... I'd have to fly to "unweight" over all the road obstacles.
Anyway, for three years I've been riding a Jamis Aurora. I really like it, and it has been great for commuting and light touring. It's main limitation, though, was in tire size. I can only fit on a 32 on the bike. I tried two brands of 35, which wouldn't fit. Also (through absolutely no fault of the bike design), I think my frame is slightly too large for me. I do not have enough seatpost showing to use a thudbuster.
My Aurora has about 10K miles on it, so it is definitely showing some component wear. I decided to buy a LHT with 26" wheels instead. (Note that the LHT with 26" wheels allow MUCH larger tires!) The geometry difference between my LHT and Aurora seems to be well within the adjustments available with seat position, stem swaps, etc. In that regard, I think they have identical fit for me, although I'm never completely done tuning my fit on any bike.
The difference in tire choices, though, is HUGE! With my LHT-26" I use a pair of 2" (55mm?) Big apples inflated only to 45 psi. They slurp up all of those road cracks that used to rattle my teeth. I could go down to 35 psi for an even smoother ride. Of course, I pay some cost in rolling efficiency. If I increase the tire pressure to 60 psi, I would pay a much smaller rolling cost but have a much harsher ride. I could also change back to much narrow tires as well, if I decided to. With these 2" tires and fenders, I have a lot of room to spare. For my next tire set, I may go up to the even more comfortable 3.15" big apples. I also now use a thudbuster, but I would definitely suggest trying larger tires first.
So for me, it was not the frame material, geometry, etc. It was just the ability to put on whooping big tires and a thudbuster.
That's been my experience with the two bikes.