The SD slot is smaller and on the left side. The CF slot is larger and is on the top.
CF cards are cheaper per MB, and are in principle faster. On the other hand, many folks like to use a wifi card or a GPS unit or a modem or some other accessory in the CF slot; then using the SD slot for memory makes more sense.
I have both kinds of cards, but I primarily use SD cards. By the way, if you choose to use SD cards, there are some brands that have known problems with the Axim -- search the site or start here
http://www.aximsite.com/boards/showt...&threadid=4031 for some information about problems with Sandisk and others.