Please advise on the mattress I require

Can please advise on the mattress best to suit me and the wife,
I'm 16 stone and the wife is 9 stone,
I sleep on my stomach and the wife sleeps on her side
I believe I would like a firm mattress? And we think we have narrowed it down to either your pocket sprung or latex mattress , we currently have a king size pocket sprung but is over ten years old!
I suffer a lot with lower back pain and believe a new mattress could help?