It would be unlikely you would get better image quality by capturing with a card with an S-video input. The video is decompressed off of the tape and then converted to analog.
Usually the lower end S-video capture cards are consumer oriented and don’t support uncompressed video capture. Decklink cards do not support capture from S-video.
If you want improved quality from DV (especially with PAL, which is 4:2:0), you can capture via SDI from a DVCAM deck that has SDI outputs to uncompressed 4:2:2 with a Decklink card. This will help you with compositing and chroma key.