Hi anpan,
All the latex you are likely to encounter has been tested for harmful substances or VOC’s through testing organizations such as Oeko-Tex or Eco-Institut whether it uses synthetic rubber, natural rubber, or a blend of both. In my experience and the experience of most manufacturers … latex is the best tolerated of all the foam materials (memory foam, polyfoam, and latex foam) for people with MCS (multiple chemical sensitivities) although there may be some who have a latex allergy (type I) or a latex contact sensitivity (type IV which is to the chemicals used in the latex formulation … usually the accelerators). There is more about latex allergies and sensitivities in post #2 here.
Beyond the specific certification there is really no way to know whether someone may be sensitive to a specific latex product but the odds are high that you will be fine with either one.
In some cases a manufacturer or retailer will send you a sample that you can use to test for any sensitivity.
Beyond the fact that one is a manufacturer and one is a retailer and the effect of multiple margins and that the material cost (the latex and the other materials such as the wool, the cover material, the inner covers etc) may be different and can have an effect on the price of the mattress, post #10 here has some of the other variables that can lead to different prices for similar products. I would always keep in mind that price is not the only factor that may be important in each person’s personal value equation although of course it usually plays a significant role in an “apples to apples” comparison.
Phoenix