S-Video looks much better on the N64 than composite. You can usually grab a multi-out composite and s-video cable for $10.
You can get one of those cables for less than $10, but be prepared to send it back. Half the time, these combo S-Video/Composite cables simply bridge composite video to luma and leave chroma disconnected, so you’re not actually getting any of the benefits of S-Video. :/
You may not notice a difference on a consumer CRT, because they (at least the TVs and some capture cards that I’ve seen) will attempt to strip chroma from luma in this case; but pro AV gear, like the Extron IN1508 (presentation switcher) or a Kramer FC-4044 (RGB decoder), that natively supports S-Video might only pay strict attention to the luma signal, which results in a black and white image (as if only luma was connected).
If you have no plans to modify your N64 for RGB and/or HDMI, buy a quality S-Video cable from someone like thefoo.83. There are different cables for NTSC and PAL consoles, so make sure you buy the one that matches your console’s region.