As much as I love the Gamecube, the PS/2 sure beats it on the sheer number of titles available.
I think you should buy the game console the same way you'd buy a computer: Decide what software you want to run, then buy the computer which runs that software.
I will say that the Gamecube's controller beats the other two by a wide margin. It's the best game controller I've ever used. I can't stand playing EA's NHL on the PS/2 because all of the buttons are the same size and arranged in a symmetrical pattern. I'm always passing when I want to shoot and vice-versa. I'm waiting for EA to port NHL to the Gamecube, I'll buy it in an instant.