With the Buffalo-IIISE, you can connect both S/PDIF and I2S/DSD at the same time, and switch between them by connecting the IP_S pads (foreground of the board picture on the web page). This will trigger the analog switches and toggle between the two inputs. For Arduino, etc, you can also trigger the switches with digital logic, by pulling the IP pin to GND.
What gives the best sonically result Spdif to the Dac board or a Spdif 2 I2s board connected to the I2s on the DAC?
I don't think you will notice a sonic difference either way. Less complexity, less wiring -> S/PDIF direct to Buffalo is what I would recommend. (Less expense as well.)
