Hi there
I don't think these days it matters all that much in terms of costs SD and XD cards are similarly priced, it's just the Sony memorystick which are very expensive ( twice the cost for the same storage)
I don't think in the short term that one type of card will become obsolete as in a VHS / Betamax way.
XD cards are supported by Olympus and Fujifilm and started in July 2002. Toshiba and Samsung manufacture the cards and are sold as Kodak, SanDisk, and Lexar.
SD cards are made by ********** (Panasonic), SanDisk and Toshiba .
It all depends on how many other devices you have which may use that type of memory. For example I have a phone which use mini SD and an mp3 player which also uses an SD card. In theory I could use the same memory card in either device rather than spend money buying a memory for each.
But memory is quite cheap these days 1Gb for �8 last time I bought one so that theory doesn't hold up nowadays.
There's a lot more info on Wikipedia if you really want to know more about each type of card and which devices use them.
SD Cards
http://en.wikipedia.org/wiki/Sd_card
XD Cards
http://en.wikipedia.org/wiki/XD_card