I personally would trade-in the X-Box because there isn't one exclsuive game on that system that I'm interested in playing.
As far as selling points for the Cube...
1. It's the least expensive of the 3 systems, currently retailing at $149.99
2. You currently have the option of getting a free GameBoy Player (allowing you to play just about every single GameBoy, GameBoy Color and GameBoy Advance game ever made), or a Free game for your GameCube
3. Free GameCube Preview Disc, featuring videos and playable demos of upcoming GameCube titles.
4. If you are into connectivity, you have the option to link a GameBoy Advance to your GameCube for use in compatible games to unlock special features.
5. Owning a GameCube allows you the opportunity to play some of the greatest games ever such as: Eternal Darkness, Super Smash Bros. Melee, Legend Of Zelda: Wind Waker, and so much more.