An Oscar is worth a LOT more than a grammy. It's a lot more common in the music industry for somebody to get a grammy then it is for somebody in Hollywood to get an Oscar.
When people think of Will in movies, they think of Will in all of these action blockbusters (I Robot, Bad Boys, Independence Day). I'm sure Will is fine with that, but I'm also sure that he wouldn't mind being taken a little more seriously as an actor. If he wins an Oscar, that's what his acting career will be remembered as. The Pursuit of Hapiness is a movie Will needs for his movie career.