The SPI library is exactly that, a library to control the SPI functions on an Atmel. The FastSPI_LED however is a library written to control a bunch of different types of LED drivers, including the WS2801. So, if you only use the SPI library, you still have to write the various functions needed to control those LED drivers. Whereas with FastSPI_LED, it's already done for you. You just call the necessary functions.