Hi larsen,
The choice between latex and memory foam would be based on personal preference. I would strongly encourage you to try some latex to make sure you prefer it over memory foam if you choose to go in that direction.
How a topper feels will change depending on the specifics of the mattress underneath it and on your body type and sleeping positions. In general though I would keep things as simple as possible and choose a single topper because it can be difficult enough to choose one topper much less how two unknown toppers will feel for you. At the very least I would choose a single topper that is as close as possible and then add a second if your experience indicates that you need a little more softness yet.
I would also keep in mind that any topper used on top of memory foam will reduce the heat that reaches the underlying memory foam which will tend to make it firmer so that’s a “moving part” than may affect your choice as well.
If your mattress is just too firm and you are just looking for some extra softness and pressure relief then the topper guidelines in post #8 here may be helpful in choosing a topper that can provide the extra pressure relief you need with the least possible risk to alignment (thicker toppers have more risk of replacing a pressure issue with an alignment issue). It also includes a link to some of the better topper sources I’m aware of.
The short answer is it doesn’t because the ILD of memory foam is not a particularly meaningful spec because it changes according to temperature, humidity, and the length of time it’s compressed and ILD testing with memory foam is not the same as the ILD testing for latex. The “feeling” of firmness of memory foam has as much to do with it’s temperature response and response time as it does with its ILD. You can read a little more about how they compare in post #2 here. The chemical formula of memory foam also changes how a particular type of memory foam feels and responds so two memory foam toppers of the same density can feel and respond differently (see post #9 here and post #8 here).
Outside of testing a specific topper on a mattress that is the same or very similar to yours … the best approach would be to use the guidelines in combination with conversations with knowledgeable manufacturers or retailers you are considering so they can provide you with more specifics and some guidance about the toppers they offer. The exchange or return policy of a topper purchase may also be an important factor to consider with a topper purchase in case you make a choice that is less than ideal.
Phoenix