Hi dzcomet,
Welcome to the Mattress Forum! 
The thickness of a mattress is just a side effect of the design and by itself isn’t particularly meaningful because whether a thicker or thinner mattress would be better or worse for any particular person will depend on the specifics of the materials (type, firmness, etc.) and on all the other layers in the mattress. Thickness is only one of many specs that are used to make different mattresses that perform and feel differently and that makes a mattress suitable for one person and not another. There is more about the effect of thickness in post #14 here. Regardless of how thick or thin a mattress may be … the most important part of the “value” of a mattress is how suitable it is “as a whole” for your particular body type, sleeping positions, and preferences in terms of PPP (Posture and alignment, Pressure relief, and Personal preferences) regardless of how thick it may be. There is certainly no reason that at your weight you would “need” more thickness than either of the mattresses you are considering but you may still “prefer” a mattress that is thicker or thinner depending on the design.
One other benefit of a thicker mattresses that have multiple layers that can be rearranged or exchanged (as you mentioned) is that there are more layering combinations possible for changing and fine tuning the performance and feel of the mattress but in many cases this wouldn’t be necessary and in some cases can lead to a level of complexity that can make predicting how the layers interact more difficult (see post #2 here).
There would be little if any practical difference between multiple layers of the same type of material, for example two 3" support layers and a single 6" support layer if they were all the same type and blend of latex and the same ILD and were inside a tight fitting cover with 3" inches of latex of the same ILD on top of them. Two 3" layers would respond a little more independently and because the elasticity of the top 3" wouldn’t be connected and “pulling back” on the bottom 3" when it compresses and “in theory” it may act a little bit softer but in practical and real life terms most people wouldn’t notice any difference in terms of performance or firmness. If a single 6" core with a 3" comfort layer was a good match for you in terms of PPP then there would be little benefit in having multiple 3" layers that were the equivalent ILD.
So the overall thickness of a mattress that is either “needed” or “preferred” would depend on the combinations of the layers and components that are needed to achieve the design goal of the mattress and provide the PPP (Pressure relief, Posture and alignment, and Personal preferences) that can best match each person and their unique body type, sleeping positions, and preferences.
And as for an 8" mattress, latex is usually poured in 6" cores at its thickest.
I hope that helps.
Phoenix