Hi boston_ma.
Welcome to our Mattress Forum! 
Just about everyone who came to this site wrestled with a similar situation and a final choice between good and good. Both of the mattresses you are considering would use good quality and durable materials with no “weak links” in their design, as I think you’re already aware, as you have a preference for latex.
Even though you are considering two products that are using good quality and durable materials the final decision, as you already discovered, may not be always easy to make as it involves a combination of objective, subjective and also intangible personal factors. I would strongly suggest that you have a good conversation with each of the online manufacturers you are considering as this can play a big role in which direction you will be leaning. Another look at your finalists from the point of view of your "personal value equation"along with the expertise and recommendations from the conversation with each of the manufacturers would most likely give you enough clues to make an informed decision. The good news is that either way it would be difficult to make a mistake.
As you are a side sleeper you can probably do well with either of the comfort options you mentioned. Only you can feel what you feel on a mattress or decide which one you would most likely prefer and that you believe is likely to be the best match for you based on your preferences and testing.
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.
Price is certainly part of your personal value equation, but again this is something that only you can decide based on all other variables that are important to you.
Reviews certainly can be helpful in learning about the reputation of a company (assuming the reviews are legitimate) and how responsive these companies are to questions or concerns, but otherwise I wouldn’t give them too much credence.
I would advise against review counting or in general using online reviews for any meaningful analysis of a product. Fortunately you already selected two quality products however I would keep in mind that the “review industry” has very aggressive ways of reaching and persuading the consumer. Because reviews have become so “important” in today’s online market they have gained a false aura of legitimacy and while they may not be the cause of the problem … many mattress companies are well aware of their ability to influence people and change buying habits and certainly take full advantage of the market mindset and target the consumer that pays significant attention to them so they are really just taking advantage of a “reality” that they know will sell mattresses for them.
I had numerous conversations with Karl ,the owner of Sleep On Latex and, he always impressed me with the integrity and stand he takes. They do not believe in "sales most of them are fake …“a product is only worth what is worth” and although they recognize that the general direction is to condition consumer behavior and encourage the “sales thinking” all the sales discounts are in effect built into the cost of the product, and in some cases that includes the cost of manipulating end user or the “professional reviewers”. Any of the Sleep on Latex reviews on their site or anywhere else are genuine with no encouragements or incentives. They are also not associated with any review sites which may account for the low numbers you are mentioning.
Latex in general is the most durable of all the foam materials (latex foam, memory foam, polyfoam) regardless of whether it is Dunlop or Talalay or is made from natural rubber, synthetic rubber, or a blend of both. There is more about the different types and blends of latex in
post #6 here ~ but the choice between different types and blends of latex is more of a preference and budget choice than a “better/worse” choice.
Zenheaven’s 100% NR Talalay from Talalay Global is a good and durable foam. The Luxury Plush Side of 1.5" 5-Zone Natural Talalay has (N1 – 14-20 ILD latex) and there are many in the industry, including one of the largest manufacturers of Talalay latex in the world, who believe that in the very softest ILD’s, Talalay latex that is made completely of NR (natural rubber) may not be quite as durable or resistant to impressions as the blended version of Talalay latex in spite of the fact that it would still have greater durability than other commonly used foams. This is because Talalay latex is lighter and less dense by nature than either Dunlop latex or firmer Talalay.
It is true that softer latex (which is generally less dense) would be a little less durable than a firmer latex (which will be a higher density) especially in the upper most layers of the mattress where it undergoes the most mechanical stress, but again I would also keep in mind that all latex is durable material compared to other types of foam and any difference in durability between different types of latex is not meaningful.
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.
As you already know Sleep On Latex is one of the manufacturing Trusted Members of this 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.
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