Hi TandL,
There is more about the different types and blends of latex in post #6 here. The 9" Plush used to be called the Tri-Comfort and uses two 4" layers of synthetic continuous pour Dunlop from Mountaintop Foam which is a good quality material and like any latex would generally be more durable than other types of foam such as memory foam or polyfoam. The synthetic latex is less dense than natural latex and would be less resilient and less “supportive” You would sink into it a little more deepy) compared to 100% natural latex in the same ILD. The Dreamfoam Freedom customizable latex mattress here uses the same material but it has three 3" layers instead of two 4" layers.
The 4" layers are also convoluted which would make them softer than a solid 4" layer of the same firmness (there is more about the effects of convoluting foam in post #2 here). The convoluting in the center of the layer is also different from the convoluting in the upper and lower third of the layers so the mattress is also zoned to be firmer in the middle under the heaviest part of the body to provide more balanced support/alignment.
Phoenix