Hi eeks,
There is more about “so called” soy foam and other “plant based” foams in post #2 here and they are just versions of polyfoam. Flex foam is just another name for polyfoam as well. There are thousands of different types of polyfoam with slight differences in their formulations, firmness, and properties but they are all just polyurethane foam (polyfoam for short).
I don’t have any way of knowing which one would be the most popular (and I personally wouldn’t choose a mattress based on its popularity anyway).
None of the mattresses you are considering have any lower quality materials or weak links that would compromise the durability or useful life of the mattress and assuming that you choose a “medium” or firmer they would all make a suitable choice for a child.
I or some of the more knowledgeable members of the site can certainly help you to narrow down your options, help you focus on better quality/value choices that are available to you either locally or online, help you identify any lower quality materials or weak links in a mattress, act as a fact check, answer many of the specific questions you may have along the way that don’t involve what you will “feel” on a mattress, and help with “how” to choose but only you can decide which specific mattress, manufacturer, or combination of materials is “best for you” regardless of the name of the manufacturer on the label or whether anyone else (including me) would have the same criteria or circumstances or would make the same choice.
I’m not going through the actual process of buying a mattress for young children (my children are grown up) so I haven’t given enough thought to the pros and cons of all the many options that are available relative to my own personal preferences and budget with the actual “intent” to make a purchase to know what I would choose but I would probably tend to prefer a mattress that includes wool quilting or perhaps alternatively add a wool quilted mattress protector (probably with a waterproof membrane) because of wool’s ability to help with humidity and temperature regulation. Once again though … I would always keep in mind that my criteria may be very different from your own (just like you may choose to eat different foods than I do or have a different definition of what you would consider to be a “healthy” diet).
Once you have narrowed down your options to a list of finalists that are all choices between “good and good” and none of them have any lower quality materials or “weak links” in their design (which they don’t) and if there are no clear winners between them (which is usually a good indication that you have done some good research) then you are in the fortunate position that any of them would likely be a suitable choice and post #2 here can help you make a final choice based on your more detailed phone conversations about each of them, the firmness and suitability of each one, their prices, your preferences for different types of materials and components, the options you have after a purchase to fine tune the mattress or exchange or return the mattress or individual layers and any costs involved (either initially or down the road if your child’s needs and preferences change), any additional extras that are part of each purchase, and on “informed best judgement” based on all the other objective, subjective, and intangible parts of your personal value equation that are most important to you.
Phoenix