Hi Mestone,
Designing and building your own DIY mattress can certainly be a rewarding and fun project and mattresses.net has some great quality/value components available but I would make sure that you’ve read option 3 in post #15 here if you are considering designing and building a DIY mattress to make sure you have realistic expectations of success and the trial and error that may be involved in finding the most suitable combination of layers and components. The “best” approach with a DIY mattress is a “spirit of adventure” where the process itself, your experiences, and what you learn along the way are more important than any cost savings which may or may not happen depending on whether you purchase any layers or components that end up needing to be replaced.
The choice between Dunlop and Talalay is a preference choice and there is more about the differences between them and how they “feel” in post #7 here but the most reliable way to know which one you tend to prefer would be based on some local testing on a range of mattresses that use each type of latex in different firmness levels. There is also more about the different types and blends of latex in post #6 here.
There are too many unknowns, variables, and personal preferences involved for someone else to be able to predict which combination of layers would be best for you based on specs (either yours or a mattress) or "theory at a distance but there are some very generic guidelines linked in mattress firmness/comfort levels in post #2 here that may be helpful. It may also be worthwhile considering a side to side split layer design because stomach sleepers will often need a firmer mattress than a side sleeper to prevent the risk of sleeping in a swayback position which can lead to alignment issues and discomfort or pain in the lower back when you wake up in the morning.
The “firmness rating” of a mattress is not particularly meaningful because there are different types of softness/firmness that people may be more or less sensitive to (see post #15 here) and what feels soft or firm to different people can be very subjective and also depends on body type, sleeping positions, and individual sensitivities and physiology. In the end … the only thing that matters is whether a mattress is “soft enough” in the comfort layers and “firm enough” in the support layers to be a good match for you in terms of PPP (Posture and alignment, Pressure relief, and Personal perferences) regardless of how firm or soft or how suitable the same mattress may be for anyone else.
Both mattresses have three layers so you would have the same number of options to rearrange the layers in both of them but since the materials are different the combinations in one will feel different from the combinations in the other. Because Dunlop is a denser material than Talalay and tends to feel firmer in the same ILD than Talalay … the 25 ILD Dunlop layer would probably feel similar in terms of firmness for most people as the 28 ILD Talalay layer (but would have a different "feel) so your first option (from the bottom up) would be somewhere in the range of firm/medium/medium layers and the second would be in the range of firm/medium/soft layers so the second one would feel a little softer … at least for most people. I would tend to lean towards firmer comfort layers for primary stomach sleepers than for primary side sleepers who generally need softer comfort layers to provide better pressure relief.
They will be closely comparable in terms of durability but they would be different in terms of firmness and “feel” for most people because of the differences between the type of latex in the layers and the firmness of each layer.
You can read more about the pros and cons of a wool quilted cover vs a thinner stretch knit cover in post #6 here. This is also a preference choice.
Phoenix