Hi eml,
You can read about how memory foam vs latex compare in post #2 here.
Unfortunately there are too many unknowns,variables, and personal preferences involved to be able to predict which topper will work best for you in terms of PPP on any specific mattress based on specs (either yours or a mattress) or "theory at a distance (see mattress firmness/comfort levels in post #2 here). The mattress under a topper can also have a significant effect on how a topper feels and performs because every layer or component in a sleeping system will have some effect on every other layer or component. In the end though slow response and temperature sensitive materials like memory foam are very different from more resilient and faster response materials so the only way to know with any certainty how well a topper will work for you on a specific mattress will be based on your own personal experience.
If you are happy with a latex topper then it would make sense to me to stick with a material that you already know works well for you but of course the type of material you choose is always a personal preference. If you do decide to choose latex and your parent’s mattress is firmer than your mattress then it may be worth considering a topper that is a little softer or thicker than the one you are currently using and if it’s a little softer then you may consider a topper that is a little thinner. If you decide that you would prefer memory foam then a 3 lb memory foam topper will be less durable and will also be less costly but it will also tend to feel softer and have less of the memory effect and in general won’t have the same level of performance and will be less “supportive” than a higher density topper and you will “go through it” more easily and feel more of the firmness of the mattress you are using below the topper.
Post #2 here and the additional topper guidelines it links to has more information about choosing a topper that can help you use your actual experience on a mattress as a guideline and reference point for choosing the type, thickness, and firmness that would have the best chance of success and links to some good sources for each of them as well but the first step is deciding on the type of material that you tend to prefer.
When you can’t test a mattress/topper combination in person … the three most important parts of choosing a topper are …
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Using the topper guidelines and your own preferences between different materials that you are familiar with to give you a reference point for the type, thickness, and firmness that has the best chance of success.
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Talking to the suppliers you are considering (assuming that they are knowledgeable about their products) so they can describe the toppers they are selling to give you a sense of how they compare to others that they sell or that you are familiar with or that are available in the market.
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Making sure you are comfortable with the return or exchange policies and the costs involved so that you have some “insurance” to offset the uncertainty and risk involved in choosing a topper that you haven’t tried in person just in case you make a choice that doesn’t work as well as you hoped for.
Phoenix