I think they are releasing Manhunt2 on the Wii first simply because Rockstar feels that if there is any console on the market that fits the gameplay mechanics of Manhunt2 to a T, it would be the Wii.
I cannot imagine how thrusting the Sixaxis around would feel more immersive than the Wii remote/Nunchuk combination, and nevermind the 360's controller.
I'm thinking Godfather: Blackhand Edition all gored up, and it would control absolutely great. Come to think of it, Splinter Cell's gameplay mechanics would also suit the Wii very well.
Gameplay-wise, I don't compare Manhunt2 to GTA at all - I couldn't stomach the first Manhunt on regular controls - I don't think the additional immersiveness of the Wii's controllers will do anything to change that!
