I don't have one but a friend has a memory foam mattress that was very expensive but she has trouble with it holding in body heat. She finds it too hot. Worth investigating but of course we live in a very hot (and getting hotter) climate here compared with you.

I love my 6 inch hand made pure cotton authentic Japanese futon. It's been great for my back but is extremely heavy. There have been times recently though where I have considered something different or even softer for the same reasons as you I suppose. I feel as if I need a little "give" and the futon doesn't give. Hard decision really.