As I think they look almost identical and a likely very similar mechanically, it's all about the feel of the soft vs. hard tail. I'm 46 and have had back issues but the hardtail on my '07 pitbull doesn't bother me. Sure, it'll jar you a little bit from time to time, but not at all a deal breaker. I guess the question is did you sell the R/B due to back issues? If so, no question go K9. I have had soft and hard tails and like them both. Damn, I'm a lot of help, ain't I?

Don't know how old you are, but if you're 50+, give a little more considerartion to the soft tail. If you're younger, pick the one that looks better to you. Whichever you choose will still be a Big Dog. A very nice problem to have
Whichever you choose, get some pics up here

Best of luck with your decision.