Atomas have diamonds in little piles. They're more precisely graded, but the piles are installed orderly (a nice thing for leveling stoned), in rows. Think of a marching band and looking across the ranks and diagonals.
You may or may not like that feel in honing something, though - especially on 400 and less - you can feel sort of a zipper type feeling. They are wonderfully flat, though, and not that expensive if you order them directly from japan (now that stu tierney has stopped, just search anywhere - ebay, etc).
DMTs always seem to have large stray diamonds on them, as do the new ezelaps, and ezelaps can have a seeming crust around the edges (that goes away, just like the large ones on the DMTs). Plastic backed DMTs are nicely flat, ezelaps are close. Both slow down more than the atomas, but that means just expect it - they last in slow mode for a long time, and if you like a 1000 grit water stone-ish kind of thing, expect it with a 600 grit ezelap and something similar for DMT - perhaps both of those will become slightly slower than a strong cutting 1000 grit stone.
(technique is still more important than all of it, aside from saying that if you're flattening stones with a diamond hone, atoma is the clear winner in my opinion. For regular sharpening, I'd prefer the ultex type stuff - double sided 8x3 for about $25US for the "DMD" branded version of those. They're not perfectly flat, but you probably won't be doing much back work on them, anyway - the finish stone can handle that, and prep for it can be done on sandpaper).
