Hello,
The main advantage that SPI has, compared with SCI, is that generally the data transfer will be much faster. But this assumes that the two MCU are typically located on the same board, or perhaps on different boards within the same equipment.
The primary advantage of SCI is that it will usually consume less pins on the MCU, and transmission distances can be much greater with the use of buffers (RS232 or RS485).
As Peg explained, the SPI slave cannot initiate a data transfer, so you might require an additional handshake connection between the two devices (in addition to slave select SS), for the slave to signal the master that it has data to send.
Another approach is to use a dual SPI master arrangement. Here, each end of the link would default to being a slave device, to be able to receive any data sent (and return response data). However, operation would revert to master operation when either end requires to initiate the sending of data.
Whether you use SPI or SCI you will likely need a basic communications protocol. This would be devised to suit the type of data being transferred.
Regards,
Mac
