Well, it just takes longer because of the total amount you're likely to be transferring to the card. The 16gb cards are no faster or slower than the 8gb. But you've effectively got 2x's the amount of data. So if it takes you 30 minutes to transfer 8gb to your card under class 2, it'll take you an hour with a 16gb card. If you have a class 4, it'll only take you 20 minutes, and class 6 will reduce that to 10, assuming all things equal of course.