Hi sweetandsourkiwi.
Welcome to our Mattress Forum! 
You selected two products using good quality and durable materials. I am sorry to hear about your preexisting condition … but you are certainly in a good position with final choices that are between “good and good”. As you probably are aware Foam Sweet Foam is one of the manufacturing Trusted Members of our site and like all the members here I think very highly of and consider them to be among the “best of the best” in the industry. They are very knowledgeable and always ready to guide their customers to make the best possible choice.
Zoning systems can certainly be useful and well worth considering for people that have more difficulty finding a mattress with the right “balance” between comfort/pressure relief (under the shoulders especially) and support/alignment (under the hips/pelvis especially) or who have more challenging circumstances or sensitivities, body types that are more difficult to “match” to a mattress, more complex medical issues, or who have a history of having more difficulty in finding a mattress that works well for them. There is more about zoning in this article and in post #11 here and the additional posts it links to but the only way to know whether any specific mattress (zoned or otherwise) will be a good match for you in terms of comfort, firmness, and PPP will be based on careful testing or your own personal experience.
The main benefit of a thicker latex mattress is that it can be more adaptable for heavier weights and multiple sleeping positions. It will compress from softer to firmer more gradually which means that there is more “range” of compression without the mattress becoming too firm for heavier weights (or parts of the body). In your case it is probably a matter of preference rather than “need”, but in the large majority of cases … 8" - 9" of latex is easily enough to include the combination softer layers (or sometimes sections) for pressure relief and firmer layers for support that most people of average or even higher weights would need.
I would be cautious about using reviews on a specific mattress (either positive or negative) or review sites in general as a reliable source of information or guidance about how you will feel on the same mattress or how suitable or how durable a mattress may be for you. In many if not most cases they can be more misleading than helpful, because a mattress that would be a perfect choice for one person or even a larger group of people in terms of comfort, firmness, and PPP (Posture and alignment, Pressure relief, and your own Personal preferences) may be completely unsuitable for someone else to sleep on (even if they are in a similar weight range). In other words … reviews or other people’s experiences, in general, won’t tell you much if anything about the suitability, quality, durability, or “value” of a mattress for any particular person (see post #13 here).
“Recommendations” do much more harm than good by encouraging consumers to believe that their recommendation or the “feedbacks” of others is all that they need to find a suitable and good quality/value mattress, which absolves the consumer of engaging in any critical thinking or educated analysis of a product.
It is important to take into account that all the layers in a mattress work together and will affect its feel and performance. All the layers and materials of the two mattresses you are considering aren’t the same, and the design is different for each mattress …. every difference between two mattresses can either have a cumulative effect or an offsetting effect that is very difficult for even a knowledgeable and experienced online manufacturer or retailer to predict how will play out for each individual. It can be surprising to many people how different they can feel (especially when most people only pay attention only to the ILD numbers and surface feel of a mattress and not to all the other specs or components that can make a significant difference in how a mattress feels and performs).
The consensus with some of the more knowledgeable people in the industry who tried the Zenheaven and with whom I spoke say the product tends to run a little bit on the firmer side as opposed to their word description of the plushness, but of course even with industry experts we have to take into consideration that it is a personal preference and opinion and that only your testing will be the most reliable way to determine suitability.
The choice between good and good is not always easy. You may wish to analyze each of your finalists through the lens of your condition and the possibility of fine-tuning the system in case your choice doesn’t turn out as well as you would have hoped …I would look at the options you have after the purchase to make changes to the mattress (either by rearranging or exchanging layers, exchanging the mattress itself, or returning the mattress for a refund and starting all over again with the insights you have gained from the “wrong” choices) can be a much more important part of a purchase. This is especially true for those that are more sensitive and have a narrower “range” for a mattress that will work well or them.
I would certainly encourage you to call each of your finalists and have a more detailed phone conversation to help “talk you through” the specifics of their mattress, the properties, the “feel”, and the construction that may be the best “match” for you based on the information you provide them and the “averages” of other customers that are similar to you. In the end, the final choice is still yours based on what you believe to be the most suitable mattress for you based on all of the parts of your personal value equation that are most important to you.
Generally giving into a sense of urgency because of “sales” may do more harm than good when it comes to suitability. Sales come and go and in my opinion they are mostly fake and are about the illusion of saving money than they are about reality. While it’s not possible to make a blanket statement about sales because each retailer or manufacturer can be different, I would keep in mind that the better manufacturers and retailers don’t generally “negotiate” (see post #6 here ) or have “fake sales” based on the time of year or holidays (see the guidelines here and post #5 here ). While they may occasionally have sales with smaller discounts or sell floor models at a reduced price, I would treat retailers or manufacturers that negotiate their prices or have “major holiday sales” with huge discounts as a red flag because manufacturers or retailers that sell good quality/value mattresses don’t need to negotiate or have “fake sales” to create a false sense of urgency and they generally sell good quality/value mattresses every day of the year at prices that are already very reasonable. Some online manufacturers do have ongoing rotating “sales” that change their name but the specifics of the sale tend to be similar.
I’m looking forward to finding out what you end up deciding … and of course, any additional comments or questions you may have along the way that I or any of the Expert members of the site can help with.
Phoenix