Hi damonmcm70,
There are hundreds of possible latex mattress designs that will vary with different types and blends of latex layers, different layer thicknesses, different firmness levels for each layer, and differences in the cover and quilting material that will all affect whether a latex mattress is a good match for you in terms of PPP (Posture and alignment, Pressure relief, and your Personal preferences) so it’s very possible that the two you tried just weren’t a good “match” for you.
This is fairly typical for major brand mattresses that all tend to use lower quality and less durable materials (in the comfort layers especially) that tend to soften or break down prematurely.
The short and simple answer …
An all latex mattress is in a completely different “category” from a more traditional innerspring mattress (see this article) and while there are hundreds or even thousands of different mattresses in each category … they are really an apples to oranges comparison and it’s unlikely that you will find a mattress in one category that feels the same as a mattress in a different category.
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” or PPP (Posture and alignment, Pressure relief, and Personal preferences) or how a mattress will “feel” to you or compare to another mattress based on specs (either yours or a mattress) or “theory at a distance” that can possibly be more accurate than your own careful testing (hopefully using the testing guidelines in step 4 of the tutorial) or if you can’t test a mattress in person then your own personal sleeping experience (see mattress firmness/comfort levels in post #2 here).
In other words … the only reliable way to know how two mattresses compare to you will be based on your own careful testing or personal experience.
The more detailed and complex answer …
There is more about the different ways that one mattress can “match” or “approximate” another one in post #9 here but it’s unlikely that you will be able to find out the specifics of the materials inside your mattress (Strearns & Foster doesn’t generally provide this information) and even if you did it’s unlikely that you will find another mattress that uses exactly the same design with the same components and materials so the only way to know whether another mattress would be similar in terms of PPP (Posture and alignment, Pressure relief, and your own personal preferences) would be based on your own careful testing (hopefully using the testing guidelines in the tutorial) or your own personal experience.
Mattress manufacturers generally try to differentiate their mattress from the mattresses made by other manufacturers and don’t normally try to “match” another mattress that is made by a different manufacturer so while you may find similar mattresses that use “similar” materials or designs and there would be many others that are in a similar general category (see this article) … it’s very unlikely that you will find another mattress that is specifically designed to “match” or “approximate” your current mattress even if they are in the same general category much less a completely different category.
The closest "aproximation to an all latex mattress that uses an innerspring would be a latex/innerspring hybrid and there is more about how an all latex mattress compares to a latex/innerspring hybrid in very general terms (there are too many variations of each one to make specific comparisons) in post #13 here and the posts it links to.
There are also no “standard” definitions or consensus of opinions for firmness ratings and different manufacturers can rate their mattresses very differently than others so a mattress that one manufacturer rates as being a specific firmness could be rated very differently by another manufacturer. Different people can also have very different perceptions of firmness and softness compared to others as well and a mattress that feels firm for one person can feel like “medium” for someone else or even “soft” for someone else (or vice versa) depending on their body type, sleeping style, physiology, their frame of reference based on what they are used to, and their individual sensitivity and perceptions. There are also different types of firmness and softness that different people may be sensitive to that can affect how they “rate” a mattress as well (see post #15 here) so different people can also have very different opinions on how two mattresses compare in terms of firmness and some people may rate one mattress as being firmer than another and someone else may rate them the other way around. This is all relative and very subjective and is as much an art as a science so the only reliable way to know whether any mattress is a suitable firmness and will be a good “match” for you in terms of PPP (regardless of the brand) will be based on your own careful testing or personal experience.
Two of the most important links in the tutorial that I would especially make sure you’ve read are post #2 here which has more about the different ways to choose a suitable mattress (either locally or online) that is the best “match” for you in terms of “comfort” and PPP (Posture and alignment, Pressure relief, and Personal preferences) that can help you assess and minimize the risks of making a choice that doesn’t turn out as well as you hoped for and post #13 here which has more about the most important parts of the “value” of a mattress purchase 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 all the parts of your personal value equation that are most important to you).
While your own careful testing or personal experience is the most reliable way to know whether any mattress is a good “match” for you in terms of comfort and PPP … when you can’t test a mattress in person then the most reliable source of guidance is always a more detailed phone conversation with a knowledgeable and experienced retailer or manufacturer that has your best interests at heart and who can help “talk you through” the specifics of their mattresses and the properties and “feel” of the materials they are 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.
Phoenix