Best mattress for us?

My husband weighs 14 stone and is 5' 10". I weigh 9.5 stone and am 5'3". We have bought a firm double mattress 18 months ago but I have been getting lower back pain and stiffness since buying this. I tend to sleep on my side. Also I think maybe we need a Kingsize bed so can move around without disturbing each other. Do we need a mattress with different springing etc on each side? I like memory foam, but my husband gets very hot. Any suggestions?