Choice of mattress, tension etc

I am looking to replace my now aging Tempur mattress. If was an older model with a soft top so I am used to softer feel. I am 6ft 3 and about 13. 5 stone. I find that firmer mattresses give me aches in my hip and shoulder as I am slim and tend to sleep on my side. I recently stayed at my mother's house and she had bought a new mattress for the guest room which I initially thought was far too soft but actually after a few days was supremely comfortable for me and I woke without and aches!What can you suggest in terms of spring tension etc?