The SPI & I²C busses are designed for local communications on a circuit board. I would use RS-485 simply because it was designed for your type of application and you can easily use it with any processor that has a UART. If you are willing to pony up for one of the selection of processors that contain a CAN Bus controller then by all means use that instead. The CAN bus is an even better choice than RS485 if you use the right driver chips and it is also suitable for exacty this type of application. I did not initially suggest CAN Bus because it limits available choices of processors and it seems like more work than necessary for your application to try to design in a separate CAN Bus controller chip. However, if you select the proper chip, you would likely find CAN Bus easy to implement (probably even easier than RS485 if you are starting from scratch). But if you already had a batch of existing software modules and/or experience with an RS485 based packet communications system then, to me, the choice of which one to use is pretty hazy.

Michael Karas