I prefer the continuous surface ones, they're faster and easier to use. the interrupted surface ones work, but are not as nice. (they're much cheaper, of course, so...)
If I were going to buy just one, it would be this one: http://www.amazon.com/DMT-D6FC-6-Inch-Dia-Sharp-Double-Sided/dp/B000GD8WHY I've had one and used it for years. I've since got 8" single sided stones in coarse and extra fine. (these: http://www.amazon.com/DMT-D8C-Dia-Sharp-Continuous-Diamond/dp/B0001WP1L0)
I use the extra fine most of the time, keeps things sharp. The 8 X 3 ones are quite a bit bigger than the 6X2, and so nicer for longer knives. There are 10" ones, if you want to spend a lot of money, and don't have to carry them around.
One thing to keep in mind when using diamond stones, any brand: they cut fast. you need to use very little pressure on them, sometimes less than the weight of the blade.
Disclaimer: I am not a honemeister and have not used the Norton flattening stone. However, I have a DMT D8C 8-Inch Dia-Sharp Continuous Diamond plate (on Amazon) that I use for flattening. I've heard it recommended a lot more often than the Norton, which I've heard some complaints about. It is twice as expensive as the Norton, but if you aren't on a super tight budget, I think it'd be worth the upgrade.