Hi amy0901,
That will really depend on how you define “better”. It would have a different “feel” from the blended Talalay in the UD latex mattress. There is more about the different types and blends of latex in post #6 here. There is also more about the differences between Talalay and Dunlop in post #7 here and synthetic Dunlop would be less “supportive” and resilient than natural Dunlop.
In terms of durability … most synthetic latex would be more durable than most polyfoam but the most important factor in the durability and useful life of a mattress is the quality of the materials in the upper comfort layers and both of them have latex in the top layers so both of them would be a durable choice for lighter body weights that don’t compress the materials below the top layer of latex. There is more about the many variables that can affect durability and the useful life of a mattress in post #4 here but none of the mattresses you are considering have any weak links in terms of durability.
In terms of safety … the synthetic Dunlop is Oeko-Tex certified which is a certification for harmful substances and VOC’s that is a little more stringent than the CertiPur certification* used for most polyfoam although both of them would most likely be “safe enough” for most people.
ADMIN NOTE: *Removed 404 link|Archived Footprint: certipur.us/pages/for-consumers/
In terms of how they “feel” and perform … the choice between different types of materials (assuming they are all good quality relative to their budget range) is more a matter of preference and budget than a “better/worse” choice.
I would also be cautious about using a soft layer of synthetic latex (or any material) in the top layer of a mattress for a child because they will generally do better with a firmer sleeping surface. If you did choose the Freedom mattress then I would use the medium on top and either put the soft on the bottom or I would remove it completely and replace it with a firm layer of polyfoam on the bottom (for the sake of cost) until when and if they need a softer layer on top of the mattress when their body matures and becomes more “curvy”.
The flex foam in the Roma is polyfoam and would be similar to the quilting polyfoam used in the cover of the Freedom mattress. There is more about the Roma here.
Phoenix