SD Card v. XD Card

whats the difference or advantage of one

No advantage either way except that there aren't any cameras made that would take both.. so it is most likely a good idea to just get the one that fits in your camera,.

... whatever that may be?

The xD is about half the size of the already small SD. I believe xD may have a smaller memory limit than SD (512Mb vs 1Gb). Transfer rates are pretty similar. SD has wider acceptance, for instance both my PDA and laptop have built in SD slots. Pity as both my cameras use CF cards.

Another point to consider: a lot of card readers support SD, but xD is rare . Both my mom and my sister use Fuji camers with xD cards, so I had to upgrade my card reader (but that was OK, since I got the opportunity to go with a USB2 reader that puts my old one to shame).

XD is actually the brain child of Olympus/Fuji.. and ONLY Olympus and Fuji use them.

They were in fact "buttheads" (technical term) to decide and create yet another memory card standard when there Pony (Smart Media) finally died. At that point they finally had the oportunity to join the rest of the free world, but instead insisted on recreating there own folly.

In this sense,.. XD is by far the "lesser" of the two standards. It is only outdone by "Memory Stick" for sheer stupidity

IMHO, the world only needs two memory formats, Compact Flash/MicroDrive and SecureDigital(SD).

CF where physical size is not an issue, SD when it is, end of story.