SPI is a close cousin of the older Microwire.
I2C is quite a bit more complex than SPI and Microwire.

There is a minimum of 3 connections for SPI and Microwire: serial clock, serial data out and serial data in. Therefore you'll see those interfaces sometimes referred to as 3-wire interfaces.

If you want to connect N devices to your microcontroller with Microwire or SPI you need to sacrifice 3+N pins to do the job. This is an area where I2C has an advantage.

Microwire and SPI shine when it comes to speed. I2C was initially specified at a maximum speed of 100kbits/sec. This was later increased to 400kbits/sec and lately some devices started to show up that boast 1Mbits/sec. This still pales in comparison to Microwire and SPI speeds. SPI has the edge over Microwire, due to the availability of higher speed peripheral devices. Today's serial EEPROM for example support up to 3MBits/s for Microwire and up to 10Mbits/sec for SPI. But even the slowest Microwire and SPI peripherals still beat the typical 100 or 400kbit/s I2C speeds.

Here is also a comparision between several serial Protocols, including I˛C, SPI and Microwire.

Hope this help.