Hi rmf5007,
The quality and durability of a mattress depends on it’s construction and on the quality of the materials inside it … not the manufacturer so I would need to know the specifics of what is inside any mattress you are considering to be able to see if there are any weak links in the mattress regardless of the brand (most manufacturers make a range of mattresses in many budget ranges that use lower and higher quality materials).
The “weak link” of a mattress is usually in the comfort layers … not the support layers … but it will also depend on the weight, weight distribution, and sleeping style of the person and how much they sink into a mattress and compress each of the layers. The position and firmness of the layers can also make a difference in durability. There is more about the factors that can affect the relative durability and useful life of a mattress in post #4 here.
In most cases having a support layer of 1.8 lb density (1.8 lb density is at the bottom of the range of polyfoam that I would consider to be good quality) over another layer of firm 1.5 lb density wouldn’t be a “weak link” of a mattress and I would be more concerned with the durability of the layers above it relative to your weight.
Phoenix