Hi digiorknow,
Welcome to the Mattress Forum! 
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 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 ).
I’m assuming that you’ve read the mattress shopping tutorial here but two of the most important links in the tutorial that I would especially make sure you 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 again nobody can speak to how any specific mattress will “feel” for someone else or whether it will be a good “match” in terms of comfort, firmness, and PPP because this is too subjective and relative to different body types, sleeping positions, and individual preferences, sensitivities, and circumstances and you are the only one that can feel what you feel on a mattress … outside of PPP (which is the most important part of “value”), the next most important part of the value of a mattress purchase is durability which is all about how long you will sleep well on a mattress. This is the part of your research that you can’t see or “feel” and assessing the durability and useful life of a mattress depends on knowing the specifics of its construction and the type and quality of the materials inside it regardless of the name of the manufacturer on the label or how a mattress feels in a showroom or when it is relatively new so I would always make sure that you find out information listed here so you can compare the quality of the materials and components to the durability guidelines here to make sure there are no lower quality materials or weak links in a mattress that would be a cause for concern relative to the durability and useful life of a mattress before making any purchase.
This model (you included specs for all of their models, two of which use memory foam) does use better quality materials (latex on each side), although I’m not sure if they use a 3" piece of the lower ILD on one side and a 3" piece of the firmer ILD on the other side, or if there are two pieces of latex on each side of varying thicknesses. Also, the polyfoam core density isn’t listed, as you mentioned, and you’d want to know that. OMF does make some mattresses using higher-quality materials.
Can you provide a link to this product? It doesn’t show on the Sleep Essentials web site, so I can’t make any meaningful comments about it. And the other link you included was for an innerspring unit from Leggett and Platt, not a box spring. This is a newer offering using steel for the edge reinforcement instead of polyfoam and is being used increasingly within the industry.
This model also uses high quality componentry, with the latex and various pocketed coil systems. I think highly of Berkeley Ergonomics and the design/components they use in their products.
All things being equal in the quality of componentry, a two-sided mattress should give you a longer comfort life than a one-sided product. In a latex configuration like the OMF, that product would be durable in both one- and two-sided configurations.
While the configurations would be different, so the feel of course would vary, there are many online options offering high-quality component latex systems. You could start with the members listed in post #21 here who are all very experienced and knowledgeable and specialize in providing the type of help and guidance on the phone that can help you make good choices. I believe that all of them compete well with the best in the industry in terms of their quality, value, service, and transparency.
If you reply back with the link to the second mattress in your list, I can complete my reply to your questions.
Phoenix