They are very different animals really, even though they both are serial.

SPI is good for fast data transfer, that's why you see it in A/D converters and the like. It needs one chip select line for each chip (with some exceptions like the68hc68p1). It's also musch easier to write code for SPI type devices, because it is a single master system (again, with some exceptions). Be careful with using SPI hardware UARTS, there can be compatibility problems, especially with the older PICs.

I2C is my preferred interface because it only needs 2 wires for lots of devices (8 bit and 10 bit addressing is possible). Code is tricker though. You can also use it in a multi master configuration which can be very useful. I2C is an open collector type of interface, so it has limited drive capability, but if you run it slow and use shielded cable, it can go a long way, look at the Philips web site for lots of info on I2C.