The tuner extracts a composite video signal from the VHF/UHF carrier frequencies for a particular channel or band. To get s-video, the composite signal has to go through a comb filter to separate the luma and chroma parts. The comb filter inside the PDI delux card is apparently pretty good, so may give better results than a filter inside a VCR.

I've not used a PC tuner in years (it was an old ATI TV Wonder). The trend these days is towards USB devices, but I don't know how good they are.