If the largest blades I'm likely to be sharpening are on the order of 4" -5" would the 6" DMT stones be sufficient? (The reason I ask is the huge price difference between the 6" and 8" stones.)
I've just begun using two of the double-sided 6"x2" Diasharp continuous stones (XC/C and F/EF). I personally feel they're plenty large enough for a fairly large blade, up to 5" or 6". Depending upon individual technique, I could see these working pretty well on larger blades too. I bought these for the same reason you mentioned, they're considerably less expensive than the larger Diasharp stones.
If the 6" stones are sufficient, are there any issues with the double sided stones from DMT? With these you could have 4 grits for a fairly reasonable price.
I have no complaints at all about my double-sided stones. In fact, from a usability standpoint, they're quite handy to just flip over and keep on going, when progressing between grits.
What about the stones like the DuoSharp series that have a non-continuous surface? I would guess that they work slower, but are they as effective?
I have two of the pocket-sized Duo-Folds, and one of the larger Duo-Sharp bench hones. They are still very effective, but the continuous surface stones from DMT will remove metal amazingly fast in comparison. It's all about the total surface area occupied by the diamonds. And, the continuous surface makes it easier to sharpen the tips of your blades, in particular. Have to be careful to avoid the holes in the interrupted surface stones (although I believe newer versions of the Duosharp bench hones have a small 'pad' of continuous surface on one end of the hone, for sharpening tips or other small tools).
Another thing, there's a 'smoothness' to the feel of the continuous hones, compared to the interrupted surface hones. Especially with the Fine/EF hone, the EF can feel almost glassy smooth under your blade. It seems hard to comprehend that diamond abrasive, rubbing against steel, can feel like this. That smoothness inspires confidence, especially when you can see how fast it works, and the hint of polish it leaves on the bevel.
I'll toss in one more thing. I've always used small, pocket-size hones for any free-hand sharpening I've done. My whole technique is built around holding the stone in one hand, and the knife in the other. I never use the hone on the bench or table. For this reason, I've really come to like the 6"x2" DMT Diasharps. They're just big enough to handle almost all typical blades, but still just small enough that I can hold it in one hand while sharpening, just as I've always done. I know it's a personal preference thing, but I really do like that.
That's my 2 cents' worth.
