Hi booknerd,
You are certainly looking at two great choices. As you mentioned both of them are members of this site which means I think very highly of them and I believe that they compete well with the best in the industry in terms of their quality, value, service, and transparency.
While they are very similar … the main difference between them in terms of their design is the type and blend of latex that each of them offer for each of their layers. SleepEZ provides a choice of either 100% natural Dunlop or blended Talalay in each of their layers while Flexus uses 100% natural Dunlop in their bottom layers and uses blended Talalay in their top layer. There is more about the different types and blends of latex in post #6 here.
There are hundreds of forum members that have purchased a component latex mattress from one or the other and are very happy with their purchase. Forum searches on SleepEZ and on Flexus (you can just click the links) will bring up many more comments and feedback about both of them.
There is more about the pros and cons of component latex mattresses with unglued layers and zip covers vs finished mattresses in post #2 here and the posts it links to.
There is also more about the pros and cons of split layers in post #2 here.
While your own careful testing or personal experience is the most reliable way to know whether a mattress is a good “match” for you in terms of comfort and PPP … when you can’t test a mattress in person then the most reliable source of guidance is always a more detailed phone conversation with a knowledgeable and experienced retailer or manufacturer that has your best interests at heart (which would include both SleepEZ and Flexus) and who can help “talk you through” the specifics of their mattresses and the properties and “feel” of the materials they are using (fast or slow response, resilience, firmness etc) and the options they have available that may be the best “match” for you based on the information you provide them, any local testing you have done or mattresses you have slept on and liked or other mattresses you are considering that they are familiar with, and the “averages” of other customers that are similar to you. They will know more about “matching” their specific mattress designs to different body types, sleeping positions, and preferences (or to other mattresses that they are familiar with) than anyone else.
Once you have narrowed down your options to a list of finalists that are all choices between “good and good” (which you have) and none of them have any lower quality materials or “weak links” in their design (which they don’t) and if there are no clear winners between them (which is usually a good indication that you have done some very good research) then you are in the fortunate position that either 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 PPP and the suitability of each one, their prices, your preferences for different types of materials, the options you have after a purchase to fine tune the mattress or exchange or return the mattress or individual layers, 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