Skeletor,
I've recently acquired all three of the products you mention. I don't know what you will be sharpening, and how polished an edge you want, but here are my impressions.
For the price, you can't do much better on the ceramic sticks, even if they can break. I've epoxied several into wooden handles and given them away. They are 3/8" BTW. They don't produce as fine a finish as I like on most knives where I sharpen the bevel, like convexed khuks or the flat-bevel "zero-edge" Scandanavian knives. They do wear in though, and get smoother, leaving a finer finish. You can take some high spots off by sanding the new ones with carborundum paper. I use mine for putting an agressive,slightly toothy edge on slicing knives or to touch up choppers before stropping. Great in the kitchen. Ditto Russ on cleaning. Haven't had one long enough to talk about wear. Finer than the fine side of the two-sided diamond plate, especially after wear-in.
The two-sided diamond plate is too small for me to use on khuks unless for touch-up by stroking the khuk with the plate. It leaves significant scratches even on the fine side. I understand that eventually some high spots will wear down and reduce this. I've used it to set bevels/edges on a couple of smaller knives and it worked well. If you already have means to set bevels/edges you might want the X-fine diamond plate instead. I've had to use carborundum paper or whetstones to get out the deep scratches after using the two-sided plate.
The viking stone is very fine, and leaves a nearly polished result. It works very well for alignining/touching-up a thin polished edge. It's quite hard and doesn't cut fast though, and I don't think I could ever remove scratches produced by the fine side of the diamond plate with it. You need to scrub it like the ceramic sticks to remove accumulated metal. I haven't tried one with oil, but water, and perhaps a little dish-soap slows down the build-up. It won't likely wear out, but I guess one could glaze it over by using the wrong kind of oil, or not ever cleaning it. I've seen three, and one has an internal flaw (they're translucent) where it might break if dropped just right/wrong, but I imagine that any could break if dropped a long way on a cement floor. Surely less fragile than the ceramic stick though. I use it to maintain polished edges on knives that I've stropped, or very finely honed and it works very well for this. On thin edges, it can also work as like a chakma, but I don't think you'll be able to do much burnishing on the hardest part of a thick khuk with it.
Windy, but hope it helps. Can't compare to similar products, it's the first diamond or ceramic stuff I've had. I like all three of them, and they are very portable, not expensive, but for me, they suppliment, not replace my waterstones, carborundum paper, chakma, and strop, at least when I have access to the latter.
EDIT:don't think you'd break the four inch stick in you pocket unless you fell just right on a rock or something. In that case, you'd probably care more about your butt/hip than the $1 stick.
