Hi BobMartin,
There is more about the differences between Talalay latex and Dunlop latex in post #7 here but I would treat the choice as strictly a preference choice rather than a better/worse choice. My daughter for example after testing both Talalay and Dunlop prefers the feel of Dunlop comfort layers (unlike her father) and each person can have very different preferences.
All the latex you are likely to encounter (either Dunlop or Talalay and made with either natural or synthetic latex or a blend of both) are certified for harmful substances and VOC’s by either Oeko-Tex or Eco-Institut and by most people’s criteria all of them would be a very “safe” material. There is more about the different types and blends of latex in post #6 here.
There is also more about organic and “safety” certifications in post #2 here and in post #2 here.
Radium is one of two companies that manufactures Talalay latex (the other is Talalay Global which used to be called Latex International).
You can see some comments about Ultimate Sleep in this topic and I would be very cautious here because I don’t believe they are selling Talalay latex.
There are too many unknowns, variables, and personal preferences involved for me to make any specific suggestions for either mattresses or toppers and each person can have very different criteria for a topper purchase based on the risk they are comfortable with and all the parts of their personal value equation that are most important to them (not to mention that the firmness of a specific mattress can also affect the thickness and firmness of a topper that works best for any particular person) … but there is more information about choosing a topper in post #2 here and the topper guidelines it links to that can help you choose the type, thickness, and firmness for a topper that has the least possible risk and the best chance for success. It also includes a link to a list of some of the better online sources for toppers I’m aware of as well (including latex).
Phoenix