I have a variety of stones for different purposes; waterstones, SiC stones, Arkansas, and diamond. I've invested a fair bit into diamond and waterstones. My diamond plates are mostly DMT Diamond, but I've also tried Lansky and Eze-Lap, I've found them all pretty similar. I use the diamond plates for flattening backs and sharpening small tools like carving chisels and knives which have a tendency to gouge my water stones. I prefer water stones for chisels and plane irons, mostly because of the surface finish I can get with them, not really because I think I'll get a sharper tool. These are Naniwa "Specialty" Stones, which are only 3/8" thick and relatively cheap. I went with these mostly to sample them but I'd happily buy the thicker versions when these ones wear out, not that they are even close to doing so. I have most of the grits from 220 to 8000 and I recommend them based on their value. If you want to avoid using water on your tools then I recommend diamonds for coarse work and arkansas for fine, followed by stropping. You can use diamond stones dry but you may want to clean with soapy water afterwards.
I'm not convinced that waterstones need flattening as often as a lot of people suggest. If you are using honing guides in the same spot then yes you probably should. I think that you should be using the stone in a way that attempts to produce uniform wear. This means not rubbing your plane iron through the middle of the stone in a straight line and instead use patterns that cover the hole stone; figure-8, small circles, diagonal, etc. Obviously it's impossible to completely avoiding wearing one spot down faster, but your pattern should also negate the effects of the low spot.