getting the right soft mattress

We have a silent night mattress at the moment which I feel is very uncomfortable. I am not getting a proper sleep. I have arthritis in my spine and problems with my legs. We have only had the mattress for two years but it is causing me more pain. It is a medium but I would like a soft mattress. Please could you tell me if pocket springs are better for a soft mattress and do less springs make a mattress softer. I will be very grateful for any advice you can give me. Sandra.