Hi Robert101.
Welcome to our Mattress Forum! 
Response to Post #17: You’ve brought up quite a few points (some of them perhaps unknowingly) and I’ll try to deal with them one by one.
I am sorry to hear that you have issues with your new Tempurpedic Breeze. Not everyone likes the feel of a memory foam mattress as it has low resilience, high hysteresis and more of an “in the mattress” feel to it that changes in response to variations in temperature, humidity (both from the ambient environment and your own body) and length of time and it is subject to compression forces. It can feel firm in certain circumstances and soft under different conditions. Due to its viscous-elastic properties and more closed celled structure, it is very good at forming a pressure relieving cradle around your body, but it will generally be the least breathable and most insulating of the foam comfort materials, so temperature, as you noticed, can be an issue. You can read more about the pros and cons of memory foam in this article
A forum search on Greensleep (Belgium manufacturer) and on Sleeptek (Canadian manufacturer) and Berkely Ergonomics (USA Manufacturer) specifically (you can just click these) will bring up more information and feedback about all of them. From a quality perspective there are no weak links in the materials of either item to sway the needle in the direction of one versus the other… and all of these mattresses use high-quality materials and could make very good choices based on the other parts of your “value equation” (see post #46 here ) that are most important to you.
Both Sleeptek and Greensleep use a Dunlop process to make their latex which is different from the Talalay process that Berkley uses. There are some videos linked in post #3 here that show the different production processes and there is also more about the differences between Talalay and Dunlop in post #7 here but you may notice a difference in feel (some people like one more over the other). Berkley Ergonomics uses European sourced Natural Talalay and pocket coils, (organic cotton, wool, and even camel down in some models) while Sleeptek and Greensleep don’t use springs. The difference between the items would be in the overall comfort and how that is achieved with the different componentry. Berkley uses the coil unit as a support layer as opposed to foam… it would be strictly a preference of your own personal PPP (Posture and alignment, Pressure relief, and your own Personal preferences).
In terms of “material value” … your choices are in the more premium end of the budget range compared to other mattresses that use similar materials but value has many components and each person has their own see personal value equation which is all the objective, subjective, and intangible factors that are the most important part of any mattress purchase. The suitability of a mattress in terms of PPP (Posture and alignment, Pressure relief, and Personal preferences), the quality of the materials and components (including the cover and quilting which can be a significant part of the cost of a mattress), the ability to customize a mattress before and after a purchase, along with the knowledge and service of the retailer you are purchasing from are all important parts of value and for many people price is less of an issue than the many other parts of “value”. The goal is always to make meaningful comparisons based on the criteria that are most important to you.
The most important “need” would be more connected with your own personal preferences and the feel that you are looking for. Making more generalized assessments of a mattress only based on one component doesn’t take into account that all the layers in a mattress work together and will affect its feel and performance so either one could be part of a mattress that provides you with your pressure relief and alignment/support needs. Assuming that all the materials in a mattress are high quality … everything boils down to which mattress design works best for the two basic functions of a mattress . Without going too much into technicalities both innersprings and a firmer latex core can be used as a core/support layer and each has very “different” characteristics. Both of them be made in many different designs and can come in softer or firmer versions with an innerspring that is firmer than a latex core or the other way around depending on the specifics of the components you are comparing.
There is more about the 4 main types of innersprings in this article and in post #10 here and more detailed information about innersprings vs latex support cores in post #2 here and more about the different types and blends of latex in this article and in post #6 here .Some of the more technical differences include …
• In general, latex will be more motion isolating than an innerspring.
• Firmer latex will be more durable than an innerspring but neither of them would tend to be the weak link of a mattress.
• Innersprings have more “air” in them so they would be more breathable than latex even though latex is the most breathable of the foam materials but the deeper layers of a mattress also have less effect on the ventilation and temperature of a mattress than the comfort layers.
• I know that this may not be of concern to you but most latex is more expensive than most innersprings.
• They “feel” very different with innersprings being more “bouncy” or “springy” than latex (although latex has more “spring” than other foam types).
• Innersprings have a more “flat line” response curve than latex ( see the graph here ) but because there are many types of innersprings with different response curves this doesn’t always hold true and there are also different types of latex which would also have different response curves.
• Innersprings absorb less energy than latex which means they are more resilient. They “push back” more strongly than latex in other words but this is not the same as softness … only about how much of the energy that is used to compress them is lost (or how high a ball will bounce when it’s dropped on them).
• Latex has a similar or higher compression modulus than most innerspring spring rates and either gets firmer with deeper compression at a similar rate as an innerspring (Talalay) or at a faster rate than an innerspring (Dunlop) which means it can be more “supportive”.
• Different innersprings have widely different abilities to take on the shape of the body (depending on the number of coils and how independently they function) while latex is much more “point elastic” than any of them because it can flex in each part of the core with less effect on the area around it than an innerspring.
There are many other more technical differences but the most important differences are the ones you can feel. Either of them can make a good choice for a support layer and in the end it really boils down to which one you tend to prefer. Both of them can provide good/support alignment. There are so many varieties of both that it’s not really possible to make more specific comparisons outside of some of the more obvious and more “generic” differences that I’ve mentioned.
I am assuming that you are asking this in connection with placing the split king on an adjustable base. As an aside, there is no single king version of power bases with the most common manufacturers and can only be ordered in a split king version, but the mattress on top of them can either be a single king-size or a split king size. The two halves of the adjustable bed can be programmed to operate together with a single remote.
As you mentioned, when you operate each side independently you would have to deal with the split in the middle and decide if feeling the split was worth the tradeoff of independent movement on each side. How much you would feel the split when the mattresses were flat would also depend on the specifics of the mattress and on how flush the top surfaces fit together. A mattress that has a more square shape with sharper (not rounded) upper edges with either latex or memory foam on top and that has no tape edge or seam on the upper edges would minimize or reduce the feel of any gap or split between them.
You can also get various types of “converters” which fill in the crack in the middle and attach the two halves together such as here and here and here and here but you would lose the ability of operating each half independently so you may be better off with a single king unless you want to keep the option open.
You could also use a single topper over both sides (preferably with the gap filled in) if the gap in between was uncomfortable but this would also eliminate the advantage of being able to adjust each side separately because it could damage the topper so it would probably be better to use a single king mattress rather than go in this direction unless again you wanted to keep the option of going back to a split king mattress open.
Having to deal with a crack in the middle that most people would feel if you tend to share or use the sleeping area in the middle for sleeping, cuddling, or other activities vs having a single sleeping surface without any gaps in the middle is one of the tradeoffs that each person would need to make for themselves to decide if the crack or gap in between was worth the benefits of having two mattresses that are independent of each other. It would really be a preference and lifestyle choice.
Hope this helps with your decision
Phoenix
Note to Twoods196: Thanks for your contribution and sharing your experience about your DIY.