If you set the Dyna up to your liking it can be that great bike for you. With also having a back problems a touring bike may be your best bet.
I gave up my cruiser looking for a Road King because I also have some issues with my back. When it came time to make the deposit I went with the SG. I can't imagine going back to a cruiser now.

 

I must add that I agree but after about 6 weeks after my back surgery, the backrest was pounding and putting pressure on my newley repaird discs so I had to remove it. Now 8 months later I am used to not having it.

 

you'll be happy with the touring bike, congratulations.

i have issues with back pain and its been a long term problem. dont be surprised if you have to make a few mods to get it perfect. the stock seat on my roadking sucked and i had to get off of it at 100 miles. i replaced it with a corbin that had a back rest and i was good till 350 miles before my back acted up. i replaced my handelbars with wild ones and now i can go 600 without any back pain.

in 07 the seats and bars were known to have issues with comfort...for some riders. im pretty sure the bars have changed, but i think the seat may be the same. just some thoughts to consider...feel well and ride safe.

 

Both really are great bikes. Both have rubber mount motors, and shocks on the rear. I'd suggest a one day rental, split the day 50/50 with a touring bike (the model you're considering, and a Dyna model. Many dealers will allow that. 1/2 day each back to back and you'll know which one fits you better. Plus many dealers will deduct the rental fee from the acquisition price if you buy from them. Check first, as this might be a selling point. Good Luck, and let us know.