DMT diamond plates wear out pretty quick. This is partly because the diamonds do wear down with use, partly because the diamond particles get dislodged from the plate. You'll only get a life time out of one, if you don't sharpen your knives very often, don't care much as your stone loses its speed, or don't live very long.
Hard knives don't necessarily wear a stone faster than softer. Tough alloys (tougher frequently goes with softer) are difficult to abrade and consequently very tough on diamond stones.
DMT labels its EF stone as "1200." God knows what rating system they use to come up with "1200," or what impression they're trying to convey when they use the number; but at an actual girt size of 9u, their "1200" is the rough equivalent of 1600 JIS.
Atoma plates last a lot longer than DMTs -- as in practically forever -- and when their performance suffers you can replace the diamond "skin," for significantly less than it would cost to replace the plate itself. But Atomas are beaucoup expensive. An 8" x 3" DMT Dia-Sharp EF runs under $60ish; the roughly equivalent Atoma 1200 is about $120, and a fresh skin costs $75.
Is the Atoma worth the extra price vis a vis the DMT? I don't use diamond plates for sharpening knives, but everyone I know (in person and online) who has used both, says the Atomas are more than worth it; for speed and feel as well as longevity.
IIRC, Ken Schwartz, who sharpens professionally, says he's still on the first set of skins for Atomas he bought a couple of years ago.
6" x 2" is an okay size for pocket knives, but not so good for longer, kitchen knives. It's not unusable, but finicky and inconvenient compared to longer stones. 8" is much better, 10" better still. Longer costs more, though. Note that 8" x 3" is twice the surface area compared to 6" x 2". That means twice the amount of diamond, which means it will do the job with far fewer strokes and a far less lower probability of error.
Personally I don't like to use diamond plates for sharpening. But there are tradeoffs to every type of stone, at the end of the day it comes down to individual priorities and tastes.
Edited by boar_d_laze - 4/11/13 at 12:44pm