OC using the multiplier would be ideal because you are not overclocking any other system components. Using the FSB OC's the RAM and PCI bus as well. That is why it is a good idea to lock your PCI bus frequency. I corrupted a SATA drive thru FSB OC one time and had to reinstall the OS. As far as the performance of the cpu goes you might get a slight advantage thru OCing the FSB due to higher bandwidth coming off the cpu, but not much if any.
I can tell you that with my GA-P35-DQ6 Mother board setup with 2X 1Gb sticks of ddr2 1066 tracers and an E4400 that, with me setting it to 9x 355 instead of 10x 330 I can see an obvious difference with the first of two winning performance wise.