For a general description of both I2C and SPI protocols and their advantages/limitations I would refer to the following article:
As for the FXOS8700CQ, you need to take into consideration that it only supports a point-to-point SPI protocol, with only one master (MCU) and one slave device (FXOS8700CQ) present on the bus. FXOS8700CQ does not tri-state the MISO pin when the CS_B pin is deasserted (logic high), which can lead to a bus conflict if multiple slave devices are present on the bus. So if you have more devices to connect, then I2C is the interface you will have to use. Also note that the FXOS8700CQ has two I2C address selection pins (SA0 and SA1) allowing connection of four FXOS8700CQ sensors on the same I2C bus.
I hope this helps.
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