I recently purchased the DLC Pocket White Balance Set at my local Creve Coeur Camera for $11.99. I wanted to test using either the grey card versus the white card to see what difference there was when choosing my white balance in Camera Raw.
I did this hand-held. As you can see below, the differences are minimal. The grey card gives me slightly warmer temperatures which to my eye are preferable. Of course you could change this later on if you used the white card.