Their is a few other ways.
You have the official component cable and or
Snes HD vision component cables (only works on pal GC)
I think hdmi looks a lot better/sharper, BUT it depends on what game and resolution
If you are running 240p by swiss then ossc with RGB cable looks a lot better.
If your running 480p then hdmi is better.
Their are plenty of comparison videos and images around the web.
@Haloharry Thanks for the answer. I would also consider HDMI to be better, but most of the stuff online that Shows the power of HDMI Shows NTSC stuff. And most NTSC games have 480p Support. Pal games does not. I just found one Video from Adam Koralik that showed 480i over HDMI and it does not look that bad, but not better than RGB. Not good enough to justify the extra cost. Maybe it Looks better when it is forced to 480p by something like the swiss mode. Not sure though.
I am quite tempted to get HDMI for the gamecube and the good Thing is that the HDMI Adapter are getting cheaper and cheaper. I will get the OSSC first and then later in the future one of the HDMI Adapters.