I'm working on a small-ish 2d fixed shoot 'em up for iPhone.

I have a master Game object that stores lives, score, current level, etc...

The game will have some other persistent attributes like Music and Sound mute state, difficulty level (easy, medium, hard).

Should I put those attributes in the Game class or should they be on their own?

What's the rule of thumb to decide how to partition attributes among classes?