manasa,
Although in many cases either will do, there are significant differences. Usually the requirements of the system will drive the design to one or the other. If this is not the case, then most likely you can use either with similar results.
some SPI advantages:
full duplex
up to 70 Mhz (but not all devices)
not limited to 8-bit words
no slave addressing
no arbitration required due to single master
some I2C advantages:
2-wire vs 4-wire (some version of SPI only require 3)
all slaves use same two wires
longer distances
communication is "acknowledged" by receiving end
multiple master is easy to implement
From experience, I can tell you that not all SPI is the same. I have run into many instances where one is not compatible with the other. For example, some SPI memory devices use a different clock polarity than others and sometimes it does not agree with the one offered in the micro.
