Hi Jewels,
The most important part of the durability of a mattress is the top 3" to 6" (depending on body type, sleeping style, and firmness and several other factors that determine how much each layer compresses when you sleep on it) so a mattress that has 2" of 1.5 lb polyfoam and say 2" of latex underneath that would generally be more durable than a mattress that uses the same 2" of 1.5 lb polyfoam and had 2" of 1.8 lb polyfoam underneath it because most people will sink into more than just the top 2" of the mattress and latex is more durable than 1.8 lb polyfoam.
While there is no way to quantify exactly how durable one mattress will be relative to another one because there are so many variables involved … there is more about the many variables that can affect the durability and useful life of a mattress relative to each person in post #2 here and the other posts it links to.
Phoenix