Hi katieweisberg,
There are too many individual variables and preferences involved for me to make specific suggestions and the “best” choice would be the one that you are most comfortable with and that you consider would be the best “value” for you. Any mattress that is in a medium firmness range or slightly firmer and that uses materials that are durable enough (which both are) and “safe enough for you” and that you are comfortable with the materials inside it would make a suitable choice for a child and as you mentioned all the materials in both mattresses are certified for harmful substances and VOC’s (Oeko-Tex for the latex and CertiPUR for the polyfoam)…
I would choose the firmness level that they suggest since they would have more experience with their mattress than anyone else.
If I was choosing this mattress I would probably ask for firm, medium, and medium layers and avoid the soft. The synthetic latex they use is Oeko-Tex certified and as you mentioned having individual layers means you could replace a single layer down the road if it becomes necessary as your child develops more adult proportions.
The question of “how safe is safe enough for me” is specific to each person or parent and their individual sensitivities, circumstances, criteria, beliefs, and lifestyle choices (just like some people may choose an organic or even a vegan diet and would consider other diets to be “unsafe” for their children) … but for most people polyfoam that is CertiPUR certified would be “safe enough” (see the children’s guidelines in post #2 here and post #2 here).
IMO … either one would make a suitable choice so your final choice (see post #2 here) will really be a matter of your own “best judgement” based on all the parts of your personal value equation that are most important to you.
Phoenix