The green cutting stones in 400 and 600 work ok but the 320 and 220 wear grooves in them very fast and IMO they aren't very well suited for using on skis. The white Alum. Ox. stones are considerably harder but the 320 and especially the 220, will still wear grooves in them. The Alum. Ox. stones I've used a lot and they work well for a variety of uses. SVST also has their T2 stones that are very hard and IMO, with regard to using actual stones, are the best available. I much prefer stones to diamond files but the thing about using stones is they require more maintenance because they get plugged up with debris much easier and they are harder to keep clean than a diamond file. Stones don't clear the debris as naturally and easily as a diamond file does and once the surface is filled up with debris on a stone, it looses its effectiveness and the longer you go before cleaning a stone, the harder it is to get it clean.
All of the different stones, files, and diamond files have their place and some certainly work better at certain things than do others.
Like anything else, using the right tool for the job is what really matters. Even though I prefer stones to diamond files, for universal use by the masses, diamond files are a better choice than are stones and Ceramic disk grinders such as the Snow Glide, this is certain to raise Jacques heart rate , is best of all. Unfortunately, they just aren't the most practical, in terms of cost.