If/when I can afford it, I would/will buy a pocket sprung mattress. My father bought one for the bed I sleep in when I visit him, because I was visiting a lot when my mother was ill and at home, and my back was bad. I absolutely love it, but have found out that good quality pocket sprung mattresses are expensive
Instead, I bought a mattress topper thing from Ikea which has helped a lot - it turns out my mattress was too hard for my ageing back! I have been choosing hard beds all my adult life, including decades sleeping on a futon, but it seems that, now I am old and overweight, I need a different sort of mattress. I had to give up on the futon some years ago because of my knees (couldn't get up from the floor, and couldn't put my socks on!
), but my bed, without the topper thingy, is still pretty hard.