I haven’t done a lot of research into crib mattresses but for me personally I would tend to focus on materials that had the least possible risk of any harmful ingredients and offgassing. This means I would focus on more natural ingredients such as coir, latex, innersprings and natural fibers or on synthetic materials that don’t offgas like PET or food grade polyethylene. I would definitely avoid anything with memory foam in it. Post #2 here and the links it contains has more about mattresses and children. I would tend to avoid polyfoam or any materials that weren’t Oeko-Tex certified (or equivalent). I would also focus on materials rather than brand just like mattresses for adults except I would add an extra margin of safety for babies and infants because their immune system isn’t as well developed.
A few comments about some that I have come across or you have mentioned …
Sealy Organic Soybean mattress:
This is not in any way organic except for the cotton cover and calling polyfoam organic is simply misleading. Soy based polyfoam simply means that a small percentage (usually under 20%) of the petrochemical polyols (which are one of the two main chemicals used to make polyfoam) has been replaced with a polyol that is derived from soy oil. You can read more about soy foams in post #2 here. I would also want to know the details of what is used in the fire barrier. I would pass this one by.
DaVinci Willow Natural Coconut Palm Organic Crib Mattress:
This also contains “latex-free foam” which means polyfoam.
It is certified by JPMA which doesn’t list any testing for harmful chemicals or offgassing.
Meets Federal Flammability Standard CFR-1632 and I’d want to know what is used in the fire barrier.
I would pass this one by as well.
Nook sleep Systems:
All the materials used in these are non toxic including the natural latex, PET plastic foam, and the fabrics which use eucalyptus viscose fiber. If these are made by Lenzing then they would also be environmentally friendly unlike much of the viscose fiber production. I would certainly consider these.
These use latex, wool, mohair, and horsetail hair all of which are “safe” materials. I would certainly consider these as well.
Naturepedic crib mattresses:
Various models use innersprings, polyethylene “springs”, polyethylene waterproof surface, organic cotton. I would consider these as well.
SleepEz crib mattress:
These are made from 100% natural Dunlop latex and 3 lbs of natural wool and the layers can be customized. These are great value and I think my thoughts would run in a similar direction to LAsleeper who has first hand experience with these (thank you LAsleeper for your comments :))
There are many more of course but these will give you an idea of the types of materials I would look for and strongly lean towards in a crib mattress and as I come across more I will add them to this list. Of course if other forum members have any they would like to add I’d welcome other options in this thread as well.