Buy the Atoma 400 and forget about DMT. The cluster pattern of the diamonds on the Atoma make it a better plate for hogging off metal and flattening stones. And buy it new, they're not that expensive. I wouldn't even consider buying a used diamond plate because you never know what you're getting. As for the longevity, well, that depends on the pressure you use, the type of steel you sharpen and/or the hardness of the stone you flatten with it. I bought a 600/1200 Atoma plate last year (I just stuck the 1200 replacement sheet on the other side of the base plate) that I use to sharpen my chisels and plane blades and it doesn't show any sign of slowing down. I also have an Atoma 1200 at home for my kitchen knives and that one also hasn't slowed down.
