Thought I'd throw in my 2-cents. Took this journey last summer. Had an old Beauty-Rest mattress that was in need of replacing. Being an old Eng I had to research this thing to death. Might say finding useful info that an eng would appreciate was hard to come by.
Didn't take long to determine a pillow-top was not in our future. Gave thought to a "memory foam" mattress. Wasn't overly impressed with what we tried out in the stores. Did a lot of reading and decided this wouldn't be our first choice. The thermoelastic foam come in different densities and each has a slightly different characteristic. Some are more sensitive to body heat and you sink in somewhat and that make for turning over or otherwise moving somewhat difficult. Wasn't impressed with what material the cover was on most Temperpedic models. See they now offer a cover that appears very close to what we ended up with.
More I read and went shopping for the more we focused on Latex. And has been mentioned in this thread not all mattresses are the same. Seems to me marketing mattresses is not unlike that of used car salesmen. One store pointed us to a brand, an old line US brand and said they had a sale going on but prices were only good for another day. Saw a cutaway model, over a ft thick and there was about 1-2 inches of latex sandwiched in the middle of cheaper poly-foam.
My advise to to NOT buy a mattress on the net unless you've actually seen one in person and at least laid on it for a few mins. People and mattresses are just too different and this is not a
one size fit all thing.
Something a little new on the market and found on a number of higher end models is Bamboo fabric covering. Very soft and helps disperse body moisture.
In the end we settled on a latex mattress, Sweda ReNu, made by Lady Americana. Come in three firmness levels, has the bamboo cover and is a sandwich of various foams of varying densities. So far, we've been very pleased. OH, on a foam mattress, one should not place it on top of a conventional box springs as they need better support.
Oh, as has been mentioned, latex come in 2 varieties, natural and Talalay. The Talalay is a little more spendy due to the mfg process. Talalay process insures a more even distribution of the foaming bubbles throughout the thickness. There is only one company in the USA that make Talalay latex.
Here is a good site for those trying to make up their minds on what kind of mattress best suits their needs.
Best Mattress Reviews