Memory cards are the “film” for your digital camera. You don’t want to have a card failure and lose your photos. A reliable brand is important.
There are other good brands but these two are very popular with working professionals due to their high reliability. I have had other brands fail but I’ve never had a Lexar or SanDisk memory card fail. Be sure you get the right kind of card for your camera (CompactFlash, Smart Media, etc.).