Are you people forgetting that if you were these people you would die excruciatingly painful deaths again and again? Crushed, blown up, falling from high places, drowned, stabbed, shot, etc etc.
But seriously, I'd be Luigi... Sitting at home with my feet up while Mario risks his life through 8 worlds of hell just for a girl who has clearly friend-zoned him. Occasionally I'd help my bro out but i'm not after Peach cause I ain't no sucka!
Also Big Boss > Solid Snake. Why be a clone when you can be the real deal.