I'm likely to get disagreement on this, but what the heck.
When shopping for diamond stones, I followed the advice of a former member here, Vasili. Before Atoma was well known, DMT was the only game in town. He liked the Diasharp line because it's continuous surface without the plastic interruptions. That being said, the Duosharp line seems like a very good value because of the two sided, two grit plates. I've been told they cut plenty fast... some even say faster than the diasharps. Either way, they seem to be a good product; I'd only be worried about digging in the point of very pointy blades.
Here's the controversial part: Vasili always said you didn't need all the grits in DMT and that skipping one in between yielded a perfectly fine finish. His progression was XXC, C, EF. I bought those plates in 8x3 diasharp and have been using them for a while with seemingly quite good results. The EF plate isn't nearly as fine as you might think, but it seems to work fine and does NOT seem to leave behind the scratch pattern from the C or the XXC.
Do you need the XXC? Honestly, it's my favorite of the 3 plates. It cuts like a demon. It's amazing. If you need to reprofile blades or quickly sharpen very dull or damaged blades, it is THE ticket.
Brian.