Killer 7 on Gamecube for sure. The loads times are about half as long, and the graphics are slightly better.
Here's a bit from the IGN review:
Still, it would've been better if more time was put into smoothing out the PS2 version. With some longer loading times with an obnoxious loop of TV static, framerate issues, hiccups, and delicate aiming that is harder to pull off with the PS2 controller, Killer 7 has made its Sony conversion a little worse for wear. The style would almost have carried this through with an equal score as the GameCube, but the fact is that these problems affect just how well the style is carried off