To make any difference at all in how a sleeper sofa sits the memory foam would have to be on both sides of the mattress. As the side of the mattress you sleep on is always folded to the inside, when the mechanism is folded up into the sofa. The reason a sleeper sofa will not sit nearly as comfortable as a standard sofa is because the mechanism has to hold the mattress as its folded up. A folded up mattress is not the same as springs. Added to that a standard sofa springs have a slight pitch down in the back, most sleeper mechanisms are designed and installed be flat with no pitch. Plus you will always be sitting on the metal linkage where you sit next to the arm. A place thats most often used when sitting on a sofa. Thats never going away on any standard sleeper, no matter of what mattress is used.

If you want a comfortable sofa sleeper to sit on, those issues must be addressed. The best way I know how is to actually drop the mechanism down inside the sofa frame about an inch and 1/2. The mechanism cant be dropped to much or it wont raise up out of the frame. Once its dropped down into the frame, then an additional 2" thick pad that fits inside; between both arms, can be added. This pad will isolate all the metal linkage next to the arms, plus add better support and comfort factor to the seat cushions.

Perhaps they've changed how sleep sofas are made, but when we bought ours 50 yrs ago at Bloomingdales, I came with two wrapped spring cushions. It was always a very comfortable sofa to sit on. We slept on it a few times, and it was acceptable for short useage.

It's now in my grandsons' bedroom and they use it when friends sleep over. It's still a comfortable sofa, and its heavy as lead!

Thanks iCustomeSofa, that's a lot of very good information. I am going to give myself a lot of time to try out various sofas. The mattress will only be for my grandchildren for 1 or 2 nights at a time.
Also, thanks Anglophilia for your input too.

Like you we are on the search for a comfortable sofa sleeper. I've read several articles that rate the best ones. Sleepers made by American Leather consistently came up on top. So hubby and I went to check them out. Very comfortable and they have 3 different types of mattresses to choose from.