What is the difference between SPI,SCI and SDI?
In the descrition of the VS1053B on Adafruit you can see that there is :


SPI flash boot for special applications
UART for debugging purposes
Serial control and data interfaces


And here there is a response about SCI : 


  SCI is not a standard term. Usually it's called a UART, and that's the
  interface you would use to communicate between the dev board and the
  PC. SPI is used for communication between chips on a board, supports
  higher speeds, and has a single master clock. UARTs are used for
  longer distances, each side asynchronously transmits their own data
  such that the clock can be recovered from it, and is usually
  transmitted at speeds just necessary for human interface devices such
  as keyboards.


So SCI is a sort of UART and you can find more infos here : http://www.ece.utep.edu/courses/web3376/SCI%20Overview.html