Hi suema,
This would be a preference choice not a “better/worse” choice. There is more about the differences between Talalay and Dunlop in post #7 here but with fibromyalgia I would tend to go with the one that was a little softer and more pressure relieving (which would likely be the Talalay).
There is more about the most effective ways to choose a mattress (either online or locally) in post #2 here that can help you identify and minimize the risks involved with each of them but when you can’t test a mattress in person then the most reliable source of guidance will always be a knowledgeable and experienced manufacturer who can help “talk you through” the different options they have available and will know much more about “matching” their different mattresses to the many different body types and sleeping styles of their customers than anyone else (including me). Like all their suggestions … your layering combination would certainly be “in the range” that was suitable for you based on the “averages” of their customers that are similar to you.
The most common reason for a mattress sagging (either visible impressions that you can see or virtual impressions that are from foam softening that you can’t) is the softening or breakdown of lower quality materials in the comfort layers which unfortunately is very common with almost all major brand mattresses. Latex is the most durable foam material in the industry so there certainly wouldn’t be a reason to be concerned with any sagging issues in a latex mattress. There is more about the many variables that can affect the durability and useful life of a mattress relative to different people in post #4 here and the posts it links to.
Phoenix