Just found this.
"Class 2 and 4 cards have better random access speed and random read/write than class 6 or 10 cards in general. To get higher sequential write speeds, manufacturers sacrifice the random access speeds. That's just how it is.
The best all around card I've ever found is Sandisk class 4.
My 32gb Sandisk class 4 outperforms any other class 10 or 2 32gb card I've tested in the random access and random read/write speeds. That's what's most important when using with WP7 or Android when using it for apps or OS files that need to be accessed quickly. Class 6 and 10 cards are setup to initialize the area of the card that is about to be used to make the sequential write/read quicker, but it takes extra time in the beginning to do the initialization. If your using the card where you need to access small bits of info from various parts of card quickly, you will do best with class 2 or 4.
Most people seem to assume that the higher the class, the better the card is for everything, but that's not how it is. Different purposes need different kinds of speed... cards with higher sequential speeds have lower random speeds."
For our purposes read speeds should be inconsequential.