Hi wssviper,
There isn’t a formula to decide on the thickness of a mattress that is best for anyone (no matter what type of mattress) and thickness by itself isn’t really an important spec in any mattress. What is in the mattress matters much more than the thickness. The only reliable way to tell is with your own personal testing for PPP.
Having said that … I would probably lean towards a little thicker at your weight because thicker mattresses have more “room” to use different layers to customize how a mattress feels and performs. There is a little more about the potential benefits of thicker layers and mattresses in post #14 here but I would use careful testing and let your body tell you which one is best for you.
In theory firmer versions of the same material will be more durable than softer versions (there is more about the factors that can affect durability and the useful life of a mattress in post #4 here) but with a 100% natural Dunlop mattress durability wouldn’t be a factor in any suitable firmness level and would be a much less important part of a choice than which one is the best match for you in terms of PPP. Any suitable firmness level would be “very durable”. There would be little benefit to choosing a mattress that would last you say 16 years over another one that would last you say 15 years (in theory only) if you didn’t sleep as well on the mattress that “in theory only” may be slightly more durable. Durability is always relative to how well you continue to sleep well on a mattress.
I wouldn’t “anticipate” foam softening and would choose the mattress that was as close as possible to your current needs and preferences with “just enough” pressure relief in all your sleeping positions and that was the best match for you in terms of PPP. If two mattresses are otherwise “equal” choices and you have no clear preference or can’t decide between them then I would choose the one that was slightly firmer but this would only be if they were otherwise equal and I really couldn’t decide between them and it seems to me that you have a clear preference between the two of them.
Phoenix