This might just be my experience, but when my mom and I stayed at SSR back in 2010, I let her have the bed, while I slept on the sofa bed. I personally liked it. I didn't give me backaches, I never had trouble sleeping (then again WDW makes you exhausted at the end of the day

), and it was comfortable. I guess it just depends on the person when it comes down to it.
Dunno if it matters which resort you stay at, whether one has better sofa beds or not, so if anyone has more info, that'd be lovely.
EDIT: Would also like to mention that my brother and his family are part of DVC, and with 3 kids, one of them about to turn 13, she usually gets the sofa bed in the sleeping arrangements. She's never complained about it. Yes yes, she's a kid at Disney, so that would have her not care so much, but she's one to point out that sorta thing if it bothers her, even at the happiest place on earth.

)