Hi halley,
the biggest problem is probably to write the client software for the SPI-slaves. I have seen many attempts to write such a client but haven't seen any working (library-)code yet.
I2C is the much simpler solution, because the Wire library already provides the neccessary infrastructure for communication using callbacks functions with the client.
Maybe you could get by with simply running the I²C-bus at a faster speed?
http://www.arduino.cc/cgi-bin/yabb2/YaBB.pl?num=1241668644
Eberhard