Hi john347:
You are correct that the latex layer in this mattress is quite firm, and it is also beneath a layer of memory foam. The comfort layers closest to your body will have the greatest impact upon your initial comfort and sensation.
In general, latex and memory foam are very different materials and can’t really be meaningfully compared to each other. It would be like asking which type of apple would be closest to an orange and the only meaningful answer would be “none of them”.
Latex is highly resilient, has a more “on the mattress” feel to it, and responds to pressure. What you feel on latex stays the same over time. You can read more about the pros and cons of latex in this article and more about the different types and blends of latex in post #6 here.
Memory foam on the other hand has very low resilience, has a more “in the mattress” feel to it, and changes its feel and response with pressure, temperature, humidity, and length of time it is subject to compression forces. It can feel firm in some conditions or circumstances and soft under different conditions. You can read more about the pros and cons of memory foam in this article and more about the different formulations of memory foam in post #8 here.
Memory foam is more motion restricting while latex is more resilient and doesn’t restrict free movement on the mattress.
Both are good at motion isolation because they are both very “point elastic” (one area can compress without affecting the area around it) but memory foam is a little better at this.
Latex is more breathable and tends to sleep cooler than memory foam.
Latex also comes in a wider range of firmness levels than memory foam and in firmer versions can be used in the deeper support layers of a mattress while memory foam is in a softer range and can’t be used in the bottom support layers of a mattress (it needs a firmer material or component underneath it). Latex in general is also a more “supportive” material (gets firmer as you compress it more deeply) than memory foam.
Latex is also a more durable material overall than memory foam although higher density memory foam is also a very durable material.
The topper guidelines in post #8 here provide some really good general guidelines for selecting a topper. From what you’ve described, it sounds like you may be looking for something that is 1"2" in latex, or 2"-3" in memory foam. There is also information in this post about the popular ILDs for latex toppers and densities for memory foam toppers. While the post is a few years old, there are also links there to some of the better topper sources of which I am aware, and their return/exchange policies. As I know you’ve already read here on the site, I can’t feel what you feel and there are multiple variables involved in making a selection, so a good conversation with the manufacturer of the topper and a review of their return/exchange plan will be important, as you stated you are not able to try out any toppers in person.
Good luck, and I look forward to finding out about your decision and the result.
Phoenix