Robert is right. Optical should be better, except toslink is fundamentally flawed. Anyone who knows the details about the SPDIF interface format and the various digital audio connection types would tell you the same. Toslink has a narrow bandwidth (6 MHz) which causes the digital pulses to ring, rather than be square. This introduces jitter in the signal (the timing between each bit is not the same, but varies). Not only that, the jitter with toslink is not random. For a PCM digital signal, this causes distortion that is easily detected with test equipment, and can be heard with good quality equipment. This is because the SPDIF format assumes that the timing between samples is exactly 1/44,100 seconds and remains constant. If the timing varies, the samples are used to reconstruct the proper amplitude of the wave, but at the wrong time, which distorts the shape of the signal, which is distortion. Unless your DAC reclocks the incoming data stream, this will effect the sound. All this has NO effect on DTS and Dolby Digital signals, because those formats are not sensitive to the timing between samples.
All this means that for DD and DTS, it probably doesn't matter which you use. For PCM data from CDs, fed to an external DAC, coax is better unless your dac reclocks the incoming signal.