Hi kate,
You are asking a more complex question than you may realize 
There are many variables that would be involved in deciding which of the two would be best for you (regardless of whether the same choice would be best for someone else) but it would really come down to which one would be the best “value” for you based on the same factors that I would use to make “value comparisons” between mattresses. There is more about the most important parts of the value of a mattress purchase in post #13 here.
The most important factor is which one would be the most suitable for you in terms of PPP (Posture and alignment, Pressure relief, and Personal preferences) and the only reliable way to know this would be based on your actual testing of the mattress/topper combination or your actual sleeping experience. Any density of polyfoam can be made in a wide range of firmness levels but as a very general guideline a 2" topper will give you a “little to a fair bit” of additional softness and pressure relief. There are some topper guidelines in post #8 here that you can work “backwards” (because most people that are choosing a topper already have a mattress not the other way around). As you can see I would choose a base mattress or foam layer that was a “little to a fair bit” firmer than you would choose if you were buying the mattress by itself. 2.6 lb polyfoam is also a higher grade of polyfoam than 1.8 lb polyfoam in some of the other more complex specs that can contribute to the suitability of a mattress such as compression modulus and resilience (see post #2 here) so this may make it a more suitable choice as well but it will always come down to which one of the two you will sleep better on in combination with your topper.
The next most important factor would be durability and for this part the 2.6 lb polyfoam would have a clear advantage over 1.8 lb polyfoam. This would be especially important if you are in a higher weight range (in the low 200’s or higher) or you have a body type where your weight is more concentrated in one part of your body because with only 2" of softer foam on top of your support core the durability of the support core would play a bigger role than it normally would because it will compress more “through” your topper when you sleep on it than it would if your comfort layers were thicker or you had a thicker topper. There is more about all the variables that can affect durability and the useful life of a mattress relative to each person in post #4 here and the posts it links to.
It may also be worth considering dragging your topper into the store so you can test the different combinations in person which would be a more accurate method of choosing than using “theory at a distance”. Other than this (and perhaps most importantly of all) I would use the knowledge, experience, and guidance of the people at Comfort Sleep Systems who are very knowledgeable and helpful and will know more about the specifics of each one and all of the mattresses they sell than anyone else.
Phoenix