The Atoma plates are roughly 1mm thick stainless steel foils with electroplated diamonds on a 10mm thick flat milled aluminium base, glued together with double sided adhesive tape. There is at least one video on Youtube which shows the process of changing the foil.
All diamond plates stay flat in their initial flatness tolerances.
I don’t know the large (8″x3″) EZE-LAP diamond plates, so I have rely on the statement of Fine Tools.
They sell both brands, the plates are roughly in the same price range, EZE-LAP is widely used and have got the reputation of being recommended by Mr. Sellers. On the other hand, Fine Tools had tested at least one EZE-LAP plate with a 0.3mm deviation in flatness. And then there are DMT Dia Sharp (as far as I know with a similar reputation as EZE-LAP) and M. Powertools advertise their “Diamond Cross 8 Inch Bench Stone 300/1000#” within 0.044mm of flatness deviation: https://www.m-powertools.com/diamond-cross-8inch-bench-stone.html
Not to mention the double sided plates sold eg. by Axminster, where it seems to be some kind of lottery to get a very flat one. But they don’t advertise “monocrystalline diamonds”, so I assume they use polycrystalline diamonds – which results in quicker wear. On the other hand, this plate costs roughly half the price of a plate by EZE-LAP, DMT or M. Powertools.
The loss of the “initial aggressivity” in my opinion is better described as “wear in”, not “wear out”. After that period diamond plates stay on a practically constant abrasive level for a long time.
In the very beginning of my woodworking hobby I had a flutter to order a double sided “Axminster Rider” plate and it turned out as good enough for my needs.
Even this “cheap” Axminster plate works fine after 3 years of constant use (and I have to admit that I worked with too much pressure for the first weeks), despite the wear in period was significantly shorter than on the Atomas.
I purchased the new plates because they cover a wider range of grit (140 to 1200 compared to 400 and 1000 of the double sided plate) and I can use them on a sharpening plate holder without having to flip them over to change the grit size.
If I had to purchase new plates by a mail order company, I’d go for the Atoma, because for me the flatness is very important and I trust the reasonable recommendation of Fine Tools.
If I had the opportunity to select plates either of EZE-LAP or DMT in a local shop and check them with a straight edge, I’d go that way.
If I’d like to have a good double sided plate, I’d purchase the M. Powertools.
When I have tight budget constraints, want to get a double sided plate and the opportunity to visit an Axminster shop to check the plates, I’d choose that option.
This just my opinion, your mileage may vary.
In other respects: what Rob wrote.
Hope that helps,