Check the LBS carrying the bikes you are interested in for rentals...I rented a Roubaix for a weekend when I was looking---a really nice ride. I ended up with a Domane (also a great ride) largely due to LBS proximity and a bit of a cost savings.

Btw, I've had back issues for many years (L4/L5...now 61yo). I know we are each different...there are things in my life that aggravate my back, but my Mustang and Domane are not on the list (noting that getting in and out of the car is where my problem is aggravated if I am not paying attention). Interestingly, I had a Moto Guzzi Sport (sportbike motorcycle) for many years and could sit it for several hours (bothered my knees, not my back LOL)...many of my Harley riding acquaintances (upright with legs thrown out in front of them putting all their weight on their tailbones) had to stop every thirty-forty minutes and get off to walk around, surprised that my back wasn't hurting...all said to suggest you may find that a endurance road bike position works better for you than expected (the plush rides of the Roubaix or Domane should be a plus). These are my opinions and experiences...others vary. Keep smiling!