PS3 Optical Out to 861 Optical In will give you standard Dolby Digital and DTS, both of which are lossy (compressed) formats.

PS3 HDMI Out to Sony TV HDMI In will give you stereo (through little speakers)

PS3 HDMI Out to HD621 HDMI In to HD621 3xSPDIF Out to 861 3xSPDIF In will give you Dolby TrueHD and DTS-MA, both of which are high resolution lossless (uncompressed) formats.

It's a little like the difference between MP3s and CDs

Bottom line - PS3 to HD621 to 861 is going to sound much better.

561, DSP5500s, DSP5000C, analog sides and rears