Whenever I suggest the Atoma 400 I get overruled by fans of the 140, but I still think they're wrong
To be fair, I've never tried the Atoma 140, and have only compared my 400 to a JKI 150 diamond plate. Please let me know if you think the Atoma is significantly different from JKI to warrant revaluation. But here's my general rationale for preferring a finer flattening plate:
If you don't flatten after every use, then a coarser plate is probably better. But I think that's bad practice, similar to not thinning knives every session. In both cases you're just kicking the can down the road, increasing your future workload and creating the need for coarser stones/plates. You can get away with a smaller kit if you're willing to put in the work every time. Your results will be more consistent and you'll be less likely to make big mistakes or remove too much material.
If you do flatten after every use, then the Atoma 400 is ideal. The slowness and suction that people complain about are actually great strengths if your main concern is removing as little material as possible. I don't bother marking Xs or checking flatness visually, I just go by feel. Once the plate slows down and starts to stick to the stone, its flat. I do this under running water and don't have problems separating them. It's not too slow if you flatten every time, each stone takes less than a minute.
Coarse diamond plates remove too much stone for my taste, and I don't like how the JKI 150 just keeps removing material without much resistance. You have to really pay attention if you only want to remove the bare minimum to achieve flatness (maybe the crisscross channels are the culprit and Atoma 140 isn't like this?) It's great for flattening a super dished stone, but overkill if you don't let your stones get super dished.
Not only do coarse plates remove more material during flattening, but also lead to more dishing during sharpening. The coarser surface texture has more dramatic peaks and valleys, and releases more mud than a finer surface would. It might be subtle but it adds up, especially for naturals if you're hoping to get a lifetime of use out of them.
/rant