Hi Automaton,
“Technically” or at least according to the PLB rating system … the Nutrition is a little firmer than the Nature. They used to have a “mattress plushness scale” on their site (you can see the page on the wayback machine here)and the Nutrition was rated as a 7 and the Nature as an 8. The Nutrition has what’s called a “dominating” layer with 3" of 27 ILD latex in the top layer over 2" of 19 ILD while the Nature has 2" of 21 ILD over 2" of 28 ILD. Both of these are on a 6" 36 ILD base layer (you can see the PLB specs here).
The Nutrition “feels” softer for many people because of the thicker comfort layers and thickness and ILD both contribute to softness. Which mattress “feels” softer can depend on body type, sleeping position, and on subjective perception.
Primary support comes from the deeper layers of a mattress so both of these have exactly the same support (36 ILD) but are different in the comfort layers which provides pressure relief and secondary support which fills in the gaps in the sleeping profile. There is more about this in post #4 here. If two mattresses are roughly “equal” then I would lean towards the firmer of the two or in this case the one that has the thinner comfort layer (even though it’s technically softer) because you can always add a topper if you need extra softness or thickness to improve pressure relief (if necessary after you have slept on it for long enough for the mattress to break in and you have a good idea of your long term sleeping experience) but you can’t firm up or remove layers that are already too soft or too thick.
There are no PLB models in the line that are in between these two.
Of course here may also be other latex mattresses made by other manufacturers available to you that are a good match for you in terms of PPP that may also be worth considering depending on the area you live.
This isn’t surprising because the quality/density of the memory foam in the Simplicity is low (around 3 lbs or less) and it would soften much more quickly … especially under heavier parts of the body like the pelvis … than higher quality/density materials.
Phoenix