Replacing a Duxiana mattress with a softer one?

needing to change our bed which is a duxiana. looking at John Lewis and Vi spring. super king size with drawers under. We are in our 70s husband 6' weighing 12 stone, I,m 5'3 and weigh 9 stone. I prefer softer husband likes firmer and has back problem. Suggestions really appreciated, we are totally confused and don't want to spend large amount of money on wrong decision.