I absolutely love the THQ XBox game. The controls 'feel' better, the graphics are photo realistic, the sound is superb (if you have a 5.1 home theater, the thunder when it's raining will have you checking your windows), and it's online multiplayer.
I bought the Namco MotoGP3 game for the PS2, and don't feel anywhere near the same. The controls are too twitchy, it doesn't feel like you have the same control of the rider, the graphics are so so, the bikes are too skinny, the lean animation is a kin to the old Sega game Hang On, the sound is lacking. I do like the power wheelies coming out of turns though. Oh, and the first person view is useless. The cockpit of the bike takes up most of the screen.