Hi karakuri,
While 7 lb memory foam would be more durable … once you are past about 6 lbs or so the durability benefits of increased density is diminished. The main benefit of a higher density foam is in its “feel” and ability to conform to the contours of the body. In many cases (including the tempurpedic 7 lb memory foam) they feel softer (once they are warm) and are more conforming than their 5.3 lb memory foams and are also more supportive (they generally have a higher compression modulus which means they become more supportive when you sink into them more deeply). Of course they are also a more costly material than lower density memory foams.
Once you have eliminated all the worst options and are down to final choices between good and good and there are no “mistakes” or “clear winners” in terms of quality or value then post #2 here is the only way I know to make final choices based on all the objective, subjective, and intangible criteria that are most important to you.
Phoenix