Hi RandRDinosaur,
I would keep in mind that here will be a break in and adjustment period for any new mattress or sleeping system as the mattress loses any of it’s “false firmness” and the cover stretches and loosens a little and the materials settle and your body gets used to a sleeping surface that is different from what it is used to (see post #3 here). This would typically be a few weeks but it can be shorter or longer depending on the specifics of the person and the mattress (higher density materials can take longer) and it can be surprising to many people how much their sleeping experience can change over the course of the first month or so. I would generally suggest sleeping on any new mattress for a few weeks at a minimum whenever possible until you are confident any symptoms are the result of the mattress rather than the initial break in and adjustment period before deciding on whether it’s a good “match” in terms of comfort, firmness, and PPP and/or deciding to exchange it.
While it’s not possible to “diagnose” mattress comfort issues on a forum with any certainty because there are too many unique unknowns, variables, and complexities involved that can affect how each person sleeps on a mattress in terms of “comfort” and PPP or any “symptoms” they experience … if after a few weeks your (or her) “symptoms” are consistent and have developed into a clear pattern then there a there is more about the most common symptoms that people may experience when they sleep on a mattress and the most likely (although not the only) reasons for them in post #2 here that may be helpful.
In very general terms … the most common (although not the only) reason for lower back pain is a mattress that is too soft (either comfort layers that are too thick/soft or a support layer or component that is too soft) and it would make sense to exchange the mattress for another one that is a little firmer.
You seem to have some good clarity about assessing a mattress for durability because as you mentioned I would have some concerns about the quality/density and durability of the layers and components in both mattresses in your higher weight range but in terms of durability they would both be suitable choices for your wife.
Unfortunately suitability and durability are different issues and while I can certainly help with “how” to choose … It’s not possible to make specific suggestions or recommendations for either a mattress, manufacturers/retailers, or combinations of materials or components because the first “rule” of mattress shopping is to always remember that you are the only one that can feel what you feel on a mattress and there are too many unknowns, variables, and personal preferences involved that are unique to each person to use a formula or for anyone to be able to predict or make a specific suggestion or recommendation about which mattress or combination of materials and components or which type of mattress would be the best “match” for you in terms of “comfort”, firmness, or PPP (Posture and alignment, Pressure relief, and your own Personal preferences) or how a mattress will “feel” to you or compare to another mattress based on specs (either yours or a mattress), sleeping positions, health conditions, or “theory at a distance” that can possibly be more reliable than your own careful testing (hopefully using the testing guidelines in step 4 of the tutorial) or your own personal sleeping experience (see mattress firmness/comfort levels in post #2 here).
Of course even careful testing isn’t a 100% guarantee of success either (although it’s more reliable than anything else outside of your own actual sleeping experience) so if you are considering two mattresses that are very similar in terms of “comfort”, firmness, and PPP and you really can’t decide between them then I would lean towards the one that is slightly firmer because you can always soften up a mattress that is too firm with a mattress pad or a topper but it’s much more difficult to firm up a mattress that is too soft without removing and replacing comfort layers that are too thick or soft or a support core that is too soft.
Phoenix