I had a quick look at your device, it says the USB input is 16 bit / 48 kHz and that the device uses asynchronous sample rate conversion and 24 bit DACs @ 192 kHz.
S/PDIF and Toslink are identical at the bitstream level and both are consumer variants of AES3 which supports 16 or 20 bit at 44.1 or 48 kHz.
the HiFace2 says it has very good clocks, which should make no difference with short cable lengths and it says it does multiple sample rates up to 192 kHz and up to 24 bits.
So if the HiFace2 is running at 192 kHz and 24 bits and toslink is 16 bit 44.1 I can imaging the s/pdif sounding best and the toslink sounding worst.
Or if you are playing out a 44.1 kHz wav file it might sound better if the Moon 10D does the sample rate conversion than if you go 44.1 to 48 on the wire and then up to 192.
Worst would be a 48 kHz file re-sampled to 44.1 in the conversion to toslink and then to 192 in the Moon.
So it is worth knowing what the number of bits and the sample rate are at each point in the chain.