Hi Frustrated Mattress Shopper,
If it was me I would make the choice based on your testing for PPP not based on durability in this case.
“In theory” thicker/softer layers of the same material would be more prone to softening than thinner layers of the same material (softening will have a greater effect over time) but you are also comparing two versions of a very durable material (relative to other materials) so with a choice between two sided mattress that are both tufted (increases durability) and use very durable materials then which one was more durable “in theory” would be a secondary factor for me because both of them would be in the “very durable” category. I would also keep in mind that durability or the useful life of a mattress is also relative to each person because a mattress that is on the edge of being too soft for a specific person will not last as long for that person as it would for someone where the same mattress was more inside the range of PPP that was suitable for them (see post #2 here).
In the end it would be a matter of your own “personal value equation” and whether price or “theoretical durability” was more important than “sleeping comfort” (assuming one was better than the other for your specific needs and preferences in terms of PPP). My own tendency would be to choose a mattress that was the best combination of PPP and durability that I could comfortably afford.
When you look back on a mattress purchase many years from now (and it would be many years in both cases if they were both well inside the range of your specific needs and preferences and neither was on the edge of being too soft for you) you will remember much more about how well you slept over the years than you will the price you paid although of course price will always play an important role in your choice.
Phoenix