Hi freddie89.
There is never a simple answer or a “formula” to determine if a mattress that has “better” specs would be suitable for you to sleep on because the specs in of themselves will not tell you too much how well you will sleep on either of the two mattresses unless you use an “all inclusive” way to assess your sleeping needs in terms of comfort, firmness, and PPP (Posture and alignment, Pressure relief, and your own Personal preferences) Both mattresses come in one firmness choice and regardless of the specs the only way to know which one would be “best” for you (regardless of whether it would be “best” for someone else) will be based on your own careful testing and personal experience as well as your history with past mattresses.
There is more about the 3 most important parts of the “value” of a mattress purchase in post #13 here which can help you make more meaningful quality/value comparisons between mattresses in terms of suitability (how well you will sleep), durability (how long you will sleep well), and the overall value of a mattress compared to your other finalists based on suitability, durability, and all the other parts of your personal value equation that are most important to you (including the specs of course and the options you have available after a purchase if your choice doesn’t turn out as well as you hoped for).
The 11" Kaya Sleep mattress has been on the market for about a year now and is one of the new entries of the one size fits all type of mattress introduced in November 2017. For normal weight ranges, this mattress this mattress would meet the durability guidelines for your BMI, but I’d place a slight caution for those of higher BMI weight ranges because of the second layer of memory foam which is 2" of 4 lbs/ft3 density. The role of the base HD Poly layer under the pocket coil would have the benefit of stabilization and giving an evenly supportive surface for the coils to sit on top and provides a protective bottom surface that makes the mattress compatible with most types of foundation designs. The 1.8 lbs/ft3 would not be of concern given that the layer is the furthest away from sleep surface.
The 12" Ojai Mattress with the top two layers using 3" at 4 lbs/ft3of memory foam would meet the durability guidelines for your normal BMI range if you like the feel of memory foam, but again it would be outside of the foam density recommendations for higher BMI ranges but. The mattress is hand tufted which also gives it more sturdiness to the entire construction.
Both mattresses are described as medium comfort choice but ultimately it would come down to the feel and type of material you like and how suitable it is for both of you in terms of comfort/support and other parts of your personal value equation.
Phoenix