Hi The_brooke,
There are too many unknowns, variables, and preferences involved for me to answer specific questions about PPP outside of more generic or conceptual information (see mattress firmness/comfort levels in post #2 here) because I can’t see you on the mattress or feel what you feel and your own careful and objective personal testing in combination with the “real life” guidance of a knowledgeable and experienced manufacturer or retailer is much more reliable than any “theory at a distance” based on specs or body type. Your ability to test for alignment is also much more reliable than any guesses based on information about body type or mattress specs.
It would also depend on the compression modulus of each foam (which is the rate that a foam becomes firmer with deeper compression) which along with some of the other foam specs can be just as important as ILD in terms of the compression properties of the foam. Higher density foams tend to have a higher compression modulus (get firmer faster with compression) than lower density foams.
Since I don’t know all the specs of each type of foam and have no personal experience with these specific combinations I would tend to go with the guidance of the manufacturer who will have more knowledge than I do about how the different foam combinations will perform together in “real life” not just based on ILD alone.
I would tend to choose the version of the 3 lb foam that was the closest to the 2.5 lb foam you tested that was most suitable for you (assuming it has the same memory foam thickness and type as you tested on top) or was a small increment in the direction of slightly softer or firmer that your testing indicated was suitable for you. To know this you would need to have more specific ILD’s for what you tested because the range you mentioned is rather wide.
I would also keep in mind that any changes to a mattress that you have tested can sometimes have a surprising effect but again the manufacturer will have more insights into whether the two foams are comparable or how they may be different.
Other than this … the most useful piece of advice I can share is that if you are truly undecided between two different firmness levels that are close together and otherwise seem to be “equal” then I would lean towards the firmer of the two because first of all mattresses tend to soften over time and it’s also easier to “fine tune” a mattress that is too firm by adding a topper than it is to make a mattress that is to soft any firmer which would require the removal and replacement of some of the foam.
Phoenix