Hi rht,
The tutorial post has much more information and guidelines for how to avoid buying a mattress that softens or sags prematurely but the most important part is to make sure you find out all the information you need about the quality of the materials in any mattress you are considering (see this article) so you can compare the the specs and the quality/density of the foams to the guidelines here to make sure that it uses high quality and durable materials and that there are no weak links in the mattress. Post #4 here and the posts it links to also have more information about the variables that can affect durability and the useful life of a mattress relative to each person.
There is also more information about choosing a mattress for those in higher weight ranges in post #3 here and the posts it links to.
This information in these links, following the steps in the tutorial post one by one, and making sure you know all the information you need to make an informed buying decision (and perhaps most importantly of all making sure you deal with knowledgeable, experienced, and transparent manufacturers or retailers that have your best interests at heart) will give you the best chance of making a great choice both in terms of PPP and in terms of durability.
Phoenix