Memory Foam/Latex or Pocket Sprung Natural Fibre

Which on should we buy and what are the benefits/drawbacks of both. We are 9.5 st an 13.5 st both expereincing back problems on our 9 year old pocket sprung bed that was about £1200 at time of purchase and I think was soft/medium. We want to buy an amazing mattress but Im feeling confused by all the information.