which is better and because transfer music from your PC via USB to mini disc? or better via optical?

There are two things being discussed here ... first is the encoding method .. each encoding method has different degrees of compression and bit rates ... generally accepted in order of quality from high to low .. SP(292 kbps) ... Hi-SP(256 kbps) .. LP2(132 kbps) ... Hi-LP/LP4(64/66 kbps) .. second is recording SP (which can only be done in real time through the digital or analog input to the recorder, as SonicStage does NOT do real SP-mode) ... for SP recording in this fashion, optical/digital output from the computer to the optical input on the MD recorder is best because it avoids two stages of conversion (from D to A, and from A to D) each of which has minor degrading effects, and analog will have added noise as well. Hence the recommendation for a digital transfer from the computer to the MD recorder for SP recording.