And wavebirds... mmm...
The GC can handle the particle effects in the game (which there are many!) better than the PS2 version. The controller works well for the way the game is set up (thanks to the large shoulder buttons), and if you want to play with 4p, the system can handle the extra users better than the ps2, too.
But other than that, the games are the same. If you really like the dual shock controller over the gc controller, you probably want the ps2 version.
Regarding finding it: I went to 5 different stores (big stores) before I found it in a small EB in a mall. Their last copy, too. You may want to call around to the different stores to ask if they have it, though, before you drive all over the place in frustration. The phone is your friend!
Barring that, just get it online. It's all over the web.