I'm under the impression that class 4 is plenty fast if you plan to put the files on the phone over USB with the card in the phone, and that anything faster is likely a waste.
If you plan to take the card out, but it into a card reader on your computer, and manage the files in windows explorer, you can maybe get a bit more speed with a higher class card.
Bottom line: Unless you want to use a class 10 in a DSLR camera in addition to your phone, there isn't really much of a reason to use anything more than a class 4 for your phone.