What mattress do you recommend for me

Looking for some guidance. I'm looking at purchasing a new mattress. I had made my mind up on a Sealy geltex pillow top mattress but I then came across your site, which by the way is excellent. I'm now quite confused to what would be best suited for myself. I now have serious doubts over the the long term durability of the Sealy. I tend to suffer on and off with upper and lower back pain/stiffness. I am approx 13 stones in weight and 5ft 10 ins in height. I am looking for something that will give good comfort but not too firm.