What type of mattress to choose?

My wife and I are trying to find a new mattress that will meet both of our needs. We currently have a mixed foam mattress on a slatted base (I covered the slats with ply wood as I was advised to do this when we bought a foam mattress). We have had this mattress about three years and I find it really comfortable but my wife has been experiencing hip and shoulder problems (lifelong medical condition) and the softness (tempur style) of the mattress seems to be making this worse. The mattress has also formed quite deep dells where we both sleep and although we have been advised this is 'normal settling' we find it difficult to turn and sleep in a different position and they are so pronounced. We have previously had a firm pocket sprung mattress and although I initially found this comfortable I found that it made my back problems worse (two lots of spinal surgery). I am 6'6" and 17.5 stone and my wife is 5'6" and about 12 stone. As you can see from my lengthy question I am really struggling to find a suitable option and was wondering whether you had any advice?