SD (secure digital) is slightly more expensive than MMC (multi media card). That is because SD cards can be up to 4 times faster than MMC cards, but this is theoretical and depends on the device that you're using it in. SD cards also have a write protection switch, kind of like a floppy disk. You probably won't notice a huge difference between the two though... I prefer the MMC cards simply because of the price, and SD cards' features aren't worth paying for IMHO.