Aren't SCART and RGB the same?
Or technically the same?
SCART is a cable that can carry several different video signals, including composite, S-Video and RGB. It is for this last one that it is still of relevance to nostalgic gamers (considering we already have perfectly-fine cables for the other two).
RGB is just a video signal: three channels, one for red, one for green, one for blue. They get combined on your TV to produce all the various colors of the screen. Most signals do not; they opt for a luminance channel (how bright/dark it is) of some sort. Composite sends it and chrominance (the color to display) along the same wire, which is why it gets so muddied (signal interference). S-Video separates those two into separate pins, which is why it looks so much better, but still has all chrominance/color information in one wire. Component separates the color information into two channels; basically, it's the red channel minus luminance and the blue channel minus luminance, and the green channel can be derived from some mathematical calculations involving those three data sets. Since the color information is now spread out so much more, it looks even better than S-Video does, but because it's getting the red, blue and green based on what luminance was, it's still not quite perfect; RGB just flat out gives the red, blue and green values, and as such is the best you could hope for.
Why would one want a XRGB as opposed to a SCART to Component adapter?
A SCART to Component adapter does not upscale the signal at all, and your HDTV's upscaler might be utter shit, producing several visual artifacts or introducing absurd amounts of input lag. An XRGB is an upscaler that is known to have minimal issues of either kind (input lag is like a frame, at worst). That said, XRGBs are costly, so if your HDTV's upscaler is
not shit, then feel free to go with that.
Also, many TVs don't support 240p over component for some stupid reason, so the SCART-to-Component adapter might not work. Caveat emptor.