If you have a Blu-ray or HD DVD player, the HDMI connection can pass an uncompressed 7.1 data stream to your compatible pre/pro. Depending on the player and the disc's soundtrack, this datastream could be 5.1 or 7.1 channels of uncompressed PCM, Dolby TrueHD, or possibly one of the DTS schemes (I can't keep 'em straight). The SP/DIF digital connection does not have the bandwidth to pass uncompressed multichannel digital sound. HDMI does.
It depends on what your source is. If you have an Oppo DV-980H, the HDMI connection can pass an uncompressed surround digital stream from DVD-A and SACD discs.
There are several Blu-ray and HD DVD players where the only way you can for sure get the new uncompressed soundtracks is via HDMI.