I'll have to teach you about google one of these days, RR.
Actually, the most common memory cards are CompactFlash (CF) and SecureDigital (SD). The Extreme Digital (xD) and Memory Stick flash memory cards are only used in products made by a few companies. xD cards are mostly only used in Olympus and Fuji devices, while Memory Sticks are mostly only used in Sony devices.
Because CF and SD cards are more widely used, they are available in the highest capacities, and the fastest write speeds sooner, and are frequently cheaper. Other formats are for narrower markets, so they cost more to manufacture.
And, yes, there are speed differences between different products and different brands, but the speed also depends on the device the card is in. There are several components that affect the apparent speed of a memory card, any one of which might be the throughput bottleneck.
Rob Galbraith's CF/SD Performance Database [ http://www.robgalbraith.com/bins/multi_page.asp?cid=6007
] shows the performance of various brands and types of flash memory products in various digital cameras. This performance database shows that the fastest cards are usually among the fastest in every camera, but will perform differently in different cameras.
And while there are a number of different ways of testing the speed of flash memory cards, there is no accepted standard that everyone uses. That's why Rob Galbraith's CF/SD Performance Database is so useful. He did all the tests, and he did them all the same way, so his results are consistent and free of bias.