Kinda timid about replying here, folks who know me might understand why...
I have an Atoma #400 of my own (and there are a few more scattered around too), a diamond plate Jon carries I believe (mine is #300/1000 and a #150 I broke out to flatten a powered wet wheel grinder) and a DMT XC/C Duo.
I've got a few other diamond things lying about as well, but they're not part of this.
Of the three, the Atoma has seen the most work. 10 times as much on stones, about the same as the DPJC (Jon's thing) on steel, and both are still quite functional. The diamonds are worn on the Atoma, but they're still present and working hard. The DPJC (screw it, it's an iWood #300, comes in #150, #800 and #150/#800 versions too) still works hard, but as I usually flatten stones with these things, I think the Atoma still has the edge in performance. The DMT Duo started out similarly to the Atoma, but has dropped off more significantly. It's still working with steel, but for stones I rarely use it. Just a right pain to use compared to the Atoma or iWood, and since I do have those two, I use them! I will say the DMT still has complete coverage of diamonds, whereas the iWood is missing a few 'pins' here and there. Of course, the DMT hasn't done 10% of the work the iWood has since I abuse the heck out of the iWood and baby the DMT.
(The DMT cost me 4 times what the iWood did/do.)
Unless the DMT is cheaper by no small amount than the other two, I'd try to avoid it. Just not confidence inspiring.
Also, if the knife is clad with something soft and sticky, I'd keep it away from diamonds entirely and use stones. Even better, a wet horizontal grinder. They're not cheap though, and overkill for a few blades here and there.
So there you go. I've used 'all of the above', I like the Atoma and iWood better because while they all work well, the Atoma and iWood do it for much longer.
Hope that helps.
Stu.
(Who'd use stones, not diamonds in this case.)