Hi Jax,
I will allow the latex expert manufacturers to weigh in on your specific question on the nuances of the two mattress proposals you have posted. A lot is going to be contingent upon your PPP’s, personal preferences, pressure relief and posture & alignment. Particularly as a 135lb side/stomach sleeper. Your firm mattress option is one that is directed to be extremely firm and perhaps that is your comfort zone. In addition to the mattress layer combination you should also be considering these fundamentals.
I would not make the ILD numbers the centerpiece of this decision. They matter, but you are choosing a sleep system, not a stack of latex specifications.
At 135 pounds and sleeping partly on your side and partly on your stomach, you should be looking just as closely at everything underneath and on top of the mattress. Your foundation can materially change how the mattress feels and performs. A rigid, closely spaced foundation will present differently than a more flexible or compliant one, and that difference can be enough to make the same mattress feel firmer or softer.
Then there is the pillow. My favorite subject. For a combination side/stomach sleeper, pillow height and resilience are particularly important because your alignment changes as you change positions. A pillow that works beautifully on your side can push your head and neck too far forward when you roll onto your stomach. Conversely, a pillow that is comfortable on your stomach may leave you under-supported on your side. What you put under your head is part of the equation, not an afterthought.
And don’t overlook the mattress protector, sheets and other bedding. A protector can noticeably change the surface feel of a mattress, and the materials used can also determine whether the sleep environment feels relatively heat-neutral or warmer. Sheets, blankets and covers can do the same thing. If you test the mattress with one setup and eventually sleep on it with a completely different setup, you may not be experiencing the same bed.
That is why I would be cautious about looking at 20 ILD versus 40 ILD and deciding that one is automatically right for you. The two constructions are substantially different, but ILD by itself does not tell you how the complete mattress system will feel under your particular body and sleeping positions.
If the manufacturer offers meaningful customization, I would also ask them specifically how they expect each construction to behave for a 135-pound combination side/stomach sleeper. They know the actual latex densities, layer thicknesses, quilting and cover materials, and how those components work together. Those details are much more informative than the ILD numbers in isolation.
One other thing I would consider is your concern about returning it. Since you already know that a return may bother you for environmental reasons, spend your return window testing the entire system exactly as you intend to sleep on it. Put it on the actual foundation, use the actual protector, pillow and bedding, and give your body enough time to distinguish “new mattress adjustment” from a genuine support problem.
In other words, don’t ask only, “Which mattress is right for me?”
Ask, “Which sleep system is going to put me in the right alignment, allow me to change positions comfortably, and maintain a sleep climate I can live with?”
That is a much harder question to answer from an ILD chart, but it is also the question that actually matters.
Just some additional food for thought.
Maverick