I was lucky enough to be in school in Golden, CO and made a trip out to the Spyderco factory outlet. Once I told them I was looking to sharpen woodworking tools, they were happy to let me root through a few boxes of unpackaged ceramic stones to find some flat ones. What I was told is that the only difference between the fine and ultra-fine white stones was that the ultra-fines were machined flat after firing. They are the exact same "grit," but the machining polishes the surface a bit. Don't lap an ultra-fine yourself, or you'll end up with a flat fine stone instead.
I ended up getting a medium (brown), a fine (white), and a leather strop with green honing compound. George's system works wonderfully (is anyone surprised?). I found the Spyderco stones to be a bit narrow, so picked up some Naniwa Super Stones for wide plane irons. They're excellent, too.
We few, we happy few, we band of brothers --
joined in the serious business of keeping our food,
shelter, clothing and loved ones from combining
with oxygen.
-- Kurt Vonnegut
Categorically better? Not sure... I like the SW8k better than I liked the Sigma 10k "back-catalogue" stone I sold you, but I'm not sure it necessarily a better stone all around... depends on what you like. Its harder and less friable, and I think I can push it further, so for me personally, yes I think the SW is better, but not necessarily for everyone.
And no, its definitely not better than the Cho10k... very very similar and very nearly as good, but not quite. Bang for buck, the SW8k is hands down better than the Cho10k, but if money were not object, I have to admit that the Cho10k gives slightly finer edge. Also, with a longer soak (15-20 minutes) the difference between the feel of the 2 stones becomes more pronounced (though still slight), and the Cho10k opens up into something extra dreamy.
Still love the SW8k, wouldn't trade is for anything, and bang for buck its pretty unbeatable.