The S-Video line connection is your answer, in this case (assuming that your HDTV has an S-Video out, not in). The HDTV signal will have already been converted to analog SDTV before it reaches the S-video or composite video out. Thus, the recording that you'd be making from a converted HDTV-to-SDTV source to VHS would have been "clearer" than if you taped it directly from an analog over-the-air channel (with all its snow and interference problems).
However, the S-Video connection will give you picture only, with no sound. Therefore, you will also need a stereo pair of audio cables (interconnects) with two RCA plugs on each end.