Hi jleb,
There is more about the most important parts of the “value” of a mattress purchase in post #13 here that can help you make more meaningful quality/value comparisons between mattresses.
There is also more information in post #2 here about the different ways to choose a mattress (either locally or online) that is the best “match” for you in terms of PPP that can help you assess and minimize the risks of making a choice that doesn’t turn out as well as you hoped for that are involved in each of them.
The “risk” of a mattress purchase and your confidence that it’s a good “match” for you in terms of PPP can be a significant part of its “value” and a local purchase that you have tested in person but is say 20% to 25% more than a very similar mattress that is sold online for many people may be equivalent “value” depending on all the parts of their personal value equation that are most important to them.
I would also make sure that you know all the specifics of all the layers and components in any mattress you purchase so that you can make more meaningful “apples to apples” comparisons between mattresses (see this article).
The main benefits of a zip cover are that it can give you access to the individual layers inside a mattress and if they are loose layers that can be easily removed (which is the case with many component latex mattresses) then you would have the ability to replace individual layers instead of the complete mattress if one of the layers softens or breaks down before the others (usually the softer top layer) or if your individual needs and preferences change over time. Of course if the internal layers are glued and they can’t be replaced individually then this benefit of a zip cover wouldn’t be important.
Secondary benefits include the ability to wash the cover (although with a suitable mattress protector this isn’t really necessary and vacuuming and spot cleaning when necessary is normally fine) and the ability to replace the cover if it wears out before the layers and components in the mattress.
There is more about the pros and cons of zip covers and loose layers vs a non zip or tape edged cover with glued layers in post #2 here and the more detailed posts it links to.
There is more about the different types and blends of latex and some of the pros and cons of each of them in post #6 here and there is more about the properties of natural vs synthetic rubber in post #2 here. All latex (Dunlop or Talalay made with natural or synthetic rubber or a blend of both) is a good quality, durable, and “safe” material relative to other types of foam materials but they each have different properties and a different “feel” so some people tend to prefer different types and blends over others either for “feel and performance” reasons of because of budget limitations (latex with higher percentages of synthetic rubber are generally less costly than latex with higher percentages of natural rubber).
Once you are down to finalists that are all choices between “good and good” and you are confident that they would all be a suitable choice in terms of PPP and none of them have any lower quality materials or weak links in their design and if there are no clear winners between them then you are in the fortunate position that any of them would likely be a suitable choice and post #2 here can help you make a final choice based on your local testing or mattresses you have slept well on, your more detailed conversations with each of them, your confidence about the suitability of each one, their prices, the options you have after a purchase to change the firmness or exchange or return the mattress, any additional extras that are part of each purchase, and on “informed best judgement” based on all the other objective, subjective, and intangible parts of your personal value equation that are most important to you.
Phoenix