I've used a blow up bed on the floor, and its quite comfy. The only problem I have with them is they are usually topped with 'velvet' type material, which I hate the feel of, even when a sheet is put over it. If you are using a topper however, it wouldn't be a problem.
I've slept on some sofa beds too and they can be ok, sometimes a little hard for my liking. Another alternative for you could be a single bed that contains another single bed underneath, can be used as a single, twin or double, but that would only be an option if you have the room.
My parents have a fold out bed, where a metal frame contains a spring base and mattress which sits neatly in the corner. I have only ever seen single ones of these, but I find the one my parents have VERY comfortable.