SCART is only on european televisions. It's better than composite, though (red/white/yellow).
The Digital AV Out thing is definitely much better than both of those, and I can't play without it anymore since I got so used to it (on a Wii--I'm actually trying to get a component cable for a Gamecube as I write this). I wouldn't get it for $100, I would look for one that's a bit cheaper than that.
Of course, this is all considering you are in North America using a North American Gamecube...
I'm sure Aleron Ives feels weird with his postcount back to <10668