The purpose of setting your WB is to measure the light balance at that moment and to correct it to neutral. This can only be done by measuring neutral tones, since that’s the only ‘color’ with a fixed mix of Red, Green and Blue AND are equal. In theory, you can use any neutral ‘color’ to set your WB.
If you measure RGB 128, 128, 150, you can tell your software/RAW converter to correct the blue color cast.
Both the Expodisk and the gray-cards have their strengths and limitations. For me in the studio, I use gray-cards, since they are the easiest to use. In the field, I sometimes use a gray-card in the scenery to make it possible to correct the WB at home on my RAW file.
Andrew Rodney recommends to use a white-card, but I think he made a little mistake here. This is why:
Your camera is not the most accurate device to measure light. It’s sensitivity decreases when the light is lower. So, measuring a whitecard is the best solution isn’t it? No, when you use a whitecard, you can clip channels without you knowing.
For example, lets say the light has a mix of colors, reading RGB 250, 253, 249. These can be corrected easily. Now you measure RGB 250, 255, 249, well this can be corrected also. In the next example, you see the problem: RGB 250, 260, 249. There is more blue, but it’s measured the same as the previous, namely RGB 250, 255, 249. This will give you an incorrect correction, ignoring the larger amount of blue in the light.
When you want to set WB, you use a graycard (or Expodisk). The standard graycards are bright enough for accurate measuring. The blackcards and whitecards are used for setting your black and whitepoints if you want (and can) preserve detail in the extremes. With digital, normally you want to avoid clipping in the whites (use RGB histogram on camera for this).