Hi tsawyer,
Although it wasn’t a “recommendation” (I leave that up to each person and their own “value equation”) … it would be my own personal preference … all other things being equal and assuming that my budget comfortably allowed it. The most important part though is that it’s the “best” for you and that you are comfortable with the tradeoff between the higher price and any differences in performance between a latex hybrid and an all latex mattress.
The difference between having 8" of latex and 9" of latex would be primarily the difference between the thickness of the softer comfort layer. The choice here is generally based on body type and sleeping positions with side sleepers generally needing a thicker comfort layer than back sleepers and stomach sleepers needing the thinnest firmest comfort layer of all. There is more about the different variables involved in this section about putting the layers together and different types of construction along with some general body type information here and some general sleeping position information here.
Because every layer works with every other layer and your choice of each layer can affect your choice of every other layer (both in terms of thickness and firmness), mattress layering can be quite complex if you go into too much detail and it’s generally best to just have a general sense of the different options and their effect so you can have a more meaningful conversation with the manufacturer rather than feeling like you need to design a mattress yourself. Most manufacturers will have a set of “average” recommendations based on your basic information (which would be suitable for most people) and if you have some feedback from personal testing on mattresses that are similar they can easily take that into account as well.
In general … most people would be fine with 8" - 9" of latex (depending on the thickness of the comfort layer they choose). there is more about the benefits of a thicker mattress and when it may be a good idea (mostly for much heavier weights) in post #14 here.
The goal of a mattress (and there is more about this in the sections I linked to earlier) is to provide the best possible combination of softness/thickness in the upper layers (for pressure relief) and firmness in the deeper layers (for support/alignment) that provides the best balance between pressure relief softness and alignment/suppport firmness for the individual person. Different body types and sleeping styles need different combinations. “Plush” pressure relieving layers may be a good option but this can still be over firmer support layers so a mattress can be both “plush” and “firm” at the same time. In general … softer/thicker comfort layers will provide a deeper pressure relieving cradle and you will sink deeper into the mattress while thinner/firmer comfort layers will provide a more shallow cradle (less pressure relief) and a feeling of being more on the mattress than in it. Different materials also play a role in this with memory foam being a more “in the mattress” and motion restricting material and latex being a more “on the mattress” and resilient feeling.
Because you are lighter than average weight … you won’t need the same degree of firmness for support as someone who was heavier (you won’t sink into softer layers as much) but you may need softer comfort layers. Side sleepers don’t need a “softer” mattress as much as they need thicker/softer comfort layers than the “flatter” sleeping positions so that the “gaps” in the side sleeping profile are filled in better and the pressure points are supporting less weight. The deeper layers can still be firm for support and “stop” the heavier pelvis/hips from sinking in further than they need to. Because all of this can vary with the different options that each manufacturer has available … once you know the basics … then a conversation with them which takes into account their greater knowledge of the specific layers and components they have available and their recommendations for someone of your body type and sleeping positions is generally the best way to make a decision about which of the layering combinations and components that they offer would be best for you.
The more personal testing you have done with different layering combinations … the more they can tailor their suggestions (within the limits of the options they have available) to your testing instead of more standard recommendations that are based on “averages” (which work well for most people may not take into account any unique needs and preferences you discover in your testing)
Phoenix