Urethane foam comes in a full range of densities and flexibilities.
From so light that a handfull seems to weigh nothing; to heavier than oak wood. It also ranges from soft and rubbery to stiff as oak wood.
Mattress made from high density urethane foam are the best sofa bed mattresses available. There are many different grades of high density urethane foam. I am referring to the premium stuff. Premium urethane foam mattresses will protect you from the bar.
Latex foam was, until recently, the very best sofa bed mattress material.
In the last few years, urethane formulations have improved so that now they yield somewhat better support and are far more durable.
Memory foam mattresses are wonderfull for thick beds. Beds over 8"thick.
They work because they conform to your body better than non memory foams. Unfortunately, they also conform to the bar. Manufacturers make a laminate of regular foam below and memory foam above. It sounds good, but sleeper sofa mattresses are only 4" thick. So these laminates offer not enough memory foam to make much of a difference, and not enough high density to unsulate you from the bar.
Springs can also be very good mattresses when the mattress is over 8" thick. Especially if they don't have to fold.
Just picture folding a slinky on itself. Not much of a slinky any more. The spring mattresses which came with Castro Convertables were complicated, uncomfortable, and deteriorated quickly. One urethane foam came on the market, it replaced spring sleeper sofo mattresses everywhere except for the very cheapest end of the scale.
Hybrids such as pillow top couch mattresses have advantages and disadvantages. We do not sell them. Its my opinion that they are problematic.