Hi JL,
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 support layer 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.
The only way to know whether any specific mattress design or combination of layers and components is a good “match” for you in terms of comfort, firmness, and PPP with any certainty will be based on your own careful testing and/or your own personal experience when you sleep on it.
I can’t speak to how any specific mattress will “feel” for someone else or whether it will be a good “match” in terms of comfort, firmness, and PPP because this is too subjective and relative to different body types, sleeping positions, and individual preferences, sensitivities, and circumstances and you are the only one that can feel what you feel on a mattress.
When you can’t test a mattress in person (doing a DYI, for example like you are) then the most reliable source of guidance is always a more detailed phone conversation with a knowledgeable and experienced supplier that has your best interests at heart and who can help “talk you through” the specifics of the properties and “feel” of the materials you are considering using (fast or slow response, resilience, firmness etc.) and the options they have available that may be the best “match” for you based on the information you provide them, any local testing you have done or mattresses you have slept on and liked or other mattresses you are considering that they are familiar with, and the “averages” of other customers that are similar to you. They will know more about “matching” their specific mattress designs and firmness levels to different body types, sleeping positions, and preferences (or to other mattresses that they are familiar with) than anyone else.
Overall, when building your own DIY mattress out of separate components that are purchased from one or several different sources then the first place I would start is by reading option 3 in post #15 here and the posts it links to (and option #1 and #2 as well) so that you have more realistic expectations and that you are comfortable with the learning curve, uncertainty, trial and error, or in some cases the higher costs that may be involved in the DIY process. While it can certainly be a rewarding project … the best approach to a DIY mattress is a “spirit of adventure” where what you learn and the satisfaction that comes from the process itself is more important than any cost savings you may realize (which may or may not happen).
Good luck!
Phoenix