I slept on the sofa bed when I was in a 2BR in SSR in may- it was one of those foam ones. It was a dedicated 2BR so it was the bed in the living room, but I assume those are the same as the ones in the studios anyway. I thought it was okay, not great. The foam is pretty firm, really nothing like sleeping on a mattress, but not terrible either. On the plus side, I was actually able to fold the bed up into the sofa with the blankets still on it so I didn't have to completely unmake and re-make the bed every morning/night.
@Tina, as far as rollaways go, I'm pretty sure Disney is completely anti-rollaway, don't even know if they exist at Disney, lol. You could always ask for all the options you mentioned though, worst they can do is say no.