Hi rob2121,
The choice between latex and memory foam is a personal preference but they are very different from each other and are very much of an apples to oranges comparison. There is more about the differences between them in post #2 here but the best way to know which one you would prefer to sleep on is your own personal testing and experience (although you are also planning to use memory foam as the top layer so it would be more comparable to a memory foam mattress, I would also keep in mind that the Alexis has polyfoam and wool in the quilting which can also affect the feel of the memory foam compared to sleeping on memory foam directly).
The thickness of a mattress and the total amount of material isn’t nearly as important as the specifics of the material inside it and how well it matches your needs and preferences in terms of PPP. Specialty foams like latex and to a slightly lesser degree memory foam are also more costly than polyfoam (although there are higher quality and more costly and lower quality and less costly grades of every type of foam material whether it’s latex foam, memory foam, or polyfoam).
To take an extreme example … if I used 20" of cheap polyfoam to build a mattress it probably would be completely unsuitable for many if not most people to sleep on … it wouldn’t be very durable at all … and it would be less costly than even 4" - 6" of higher quality specialty foams used in just the comfort layers of a better quality mattress.
Just in case you haven’t done it already … I would also make sure you have a more detailed phone conversation with any online manufacturer you are considering so that they can give you the benefit of their knowledge and experience to help you decide if one of their mattresses would be a suitable choice for you.
Phoenix