Hi egodestruct,
The best way to make apples to apples comparisons between mattresses is based on their construction and the materials and components inside them. The name of the manufacturer on the label is much less important than the construction and materials in the mattress and mattress reviews will not give you much meaningful information about the quality, value, or suitability of a mattress (see post #13 here about reviews).
As you mentioned the Ethos is one of the brand names manufactured by Diamond Mattress. You can see the details of the mattress here. It uses 8" of 100% natural Talalay latex with 7 lbs of wool in the quilting and it is tufted instead of glued which precompresses the materials to help prevent impressions and keep the layers from shifting. This is a very good quality mattress with no “weak links” in its construction. you can see some comments about Diamond Mattress in post #2 here but in this case you would know all the layers and materials in the mattress so the comments about not being able to find out the specifics of what is in their mattress wouldn’t apply here.
The closest comparison in terms of the amount and type of materials in the SleepEz line would be the SleepEz organic 9000 which also uses 8" of 100% natural Talalay latex in its construction and has a wool quilted cover. The difference is that there is less wool in the quilted cover and it has 3 loose layers (two 3" layers and one 2" layer), has a zip cover so you can access the layers, and has options that you can choose to customize the firmness of the mattress in each layer either before a purchase or by re-arranging or exchanging layers after a purchase as well. There is no tufting because this would prevent you from removing or exchanging the layers.
While the construction is similar and the amount and type of the latex is the same … they would have a different “feel” in terms of PPP (Posture and alignment, Pressure relief, and Personal preferences) because of the differences in their construction and design and any differences in the firmness of the latex in each one. Tufting will also be firmer than loose layers of the same firmness levels and more wool will have a different “feel” as well.
SleepEz is one of the members here which means I believe they compete well with the best in the industry in terms of quality, value, and service. A forum search on SleepEz (you can just click this) will also bring up hundreds more posts with more information and feedback about them.
Which one is “better” for you would depend on which of the factors in your personal value equation are most important to you but both of them would make good quality choices and you would be choosing between “good and good” in a final choice between them.
I don’t think this is a realistic concern and you can see my reply to the same question that read the same review you probably did in post #2 here. With a single layer there would not be any transition in the middle from one firmness level to the other (or you would feel it much less if the deeper layers had a different firmness level) but you wouldn’t feel the seam itself in either case.
Phoenix