Hi paddyb,
Again … the density of the memory foam or polyfoam in the specific product you are purchasing is what you need to know because it may be different than other products that use memory foam with the same name…
Latex and memory foam are very different materials with very different properties but assuming that the memory foam is a good quality/density material the choice between them is more of a preference and budget choice than a “better/worse” choice. There is more about some of the differences between memory foam and latex in post #2 here. Some people tend to prefer the faster response and more resilient and “on the mattress” feel of latex and some prefer the slower response and more “in the mattress” feel of memory foam but the best way to know which type of materials or mattresses you tend to prefer in general terms would be based on your own careful testing and/or your own personal experience with each material in a range of different firmness levels.
Phoenix