I'm yet to find a comfy sofa bed - if we stay somewhere with that as the only option I tend to take the mattress off the bed and put it on the floor. Metal bars are not my friend! We have one, and dh reckons it's not too bad, but we haven't actually had to use it yet for an overnight guest so I don't know how good it is lol
We have a futon in our study and I've slept on it a few times. It's not fantastic but it's not as bad as sofa beds I've endured either. It's a good quality futon though - not a pipe one with mattress (used one of those as my bed at "home" when I visited for four years while I was at uni - this is a definite improvement on that one.

We generally give people the option of sofa bed in lounge, futon in study or mattress on the floor in spare room, and most often they choose mattress on the floor . Based on that I'd suggest getting a couch that you find comfortable and will use, as that will be primary purpose, and a decent inflatable air bed (you can get ones that go up and down in a couple of minutes).