Hi cwalters,
Just in case you haven’t read it yet … I would make sure you’ve read the tutorial post here which has all the basic information, steps, and guidelines that can help you make the best choices … and know how to avoid the worst ones.
So called “soy foams” or other “plant based foams” are just polyfoam that has replaces a small percentage of one of the chemicals used to make foam (the polyol) with a plant based derivitive. You can read more about them in post #2 here. I would consider these to be closely comparable to regular polyfoam and would compare them based on their density and of course on how they feel and perform in terms of PPP.
Any type of foam can be made in softer or firmer versions.
All latex is a high quality and durable material relative to other types of foam although softer versions of any material will be less durable than firmer versions (you can read more about the factors involved in durability in post #4 here and the other posts it links to). The “weak link” of a mattress is generally in the comfort layers and not the deeper layers of a mattress.
I would consider the choice between Dunlop and Talalay as a preference choice and not a “better worse” choice. Some people prefer one and some the other and both are very comparable in terms of durability. Both have a long history of use in mattresses that have lasted for decades. There is more about the different types and blends of latex in post #6 here and more about how Dunlop and Talalay compare in terms of “feel” in post #7 here but your own personal testing and experience is the most reliable way to know which one you prefer.
Phoenix