Hi kme,
Just in case you haven’t read it yet … the tutorial post here has the basic information, steps, and guidelines that can help you make the best possible choices.
You can read a little more about plant based polyfoams or memory foams (including soy and castor) in post #2 here and about Acella-flex in post #4 here. In general they are a small step in a “greener” direction but all polyfoam is primarily a petrochemical product.
Both Arizona Premium and Rocky Mountain Mattress are members of this site which means I think highly of both of them and they both compete well with the best in the industry in terms of quality and value but Arizona Premium tends to specialize more in latex mattresses while Rocky Mountain tends to specialize more in memory foam mattresses. As in any mattress purchase … you would need to talk with each of them to find out the specific density of the memory foam and polyfoam layers along with the other parts of your personal value equation to be able to make meaningful comparisons between them.
There is a list of some of the better online memory foam retailers and manufacturers I’m aware of linked in the tutorial post but Rocky Mountain is the only one that offers a range of “kit” memory foam mattresses where you can choose individual comfort and support layers in a memory foam mattress.
If you decide to go in a complete DIY direction where you buy all the separate layers individually (rather than as a kit from a single manufacturer) I would read post #15 here first which also has a link to some of the better options I’m aware of for individual layers and components.
Phoenix