It's difficult to find a modern LCD monitor that can display a 15 kHz signal through VGA, so I'm wondering if S-Video is just as good when displaying 240p and 480i? I've looked at screenshot and video comparisons online, but I can't see a difference.
S-Video relies on colour transformation from RGB to YUV, and then takes the U and V and modulates them using a colour subcarrier. The TV has to undo all these steps in order to get the original RGB signal. If the subcarrier frequency is not in phase with the pixel clock (as will be the case if using different crystals), then moving artifacts will show up in the final image. Those moving artifacts are of course not visible in a screenshot.
There are upscalers that can take a 15kHz RGB signal (as generated by the Commodore Amiga, for example) and convert it into HDMI along with the audio. There are similar converters that do the same from RGB to VGA. Look for "arcade rgb to vga"
The answer will depend a lot on what application you have in mind. There is a clarity difference, but how much it will matter will depend on how large of a display the output is going to as to how much you'll notice. On a 20" desktop monitor, you probably won't notice a huge difference, on a 50" TV the difference will be more pronounced.
The other factor that may matter depending on what you are using it for, is displaying 80 column text. You stated 240p or 480i, but you didn't specify horizontal resolution. If you're using, for example, a 640 x 240p resolution and dealing with a lot of text display (say application use over gaming), the difference will matter more. 80 column text is certainly readable on S-Video but the clarity will be better on a 15khz VGA signal if that's going to be a primary use for it.