First of all, S-Video, as opposed to RCA/COMPOSITE connections only really has one difference. The luminance and chrominance are split in to two pins (aka they're seperated), preventing crosstalk and such. A lot of the source stuff, especially for things like laserdisc and vhs video are actually recorded in composite video. This means, if they do have an s-video output, it's actually going through a comb filter first, bypassing any comb filter your television may have.
TVs also have comb filters that can seperate the chrominance and luminance in composite connection, but most of them just don't do as good as the s-video output on your game or dvd player. This may be because there are several different types of comb filters in televisions. There are three main types: 2-line, 3-line, and 3D YC. I believe there are a few non-standard types, like a 4d one as well.
While I've never owned a tv that had that great of comb filter apparently (because the s-video was always better than composite on the ones i've owned), if you have a good tv, you should try both and compare. You might just have one with a good comb filter, never know. Hope this helps.