Depends on how visible they'll be and how fully the characters are fleshed out. If they're fleshed out very well and it can be said that they have a set personality and history that doesn't require the player to fill in the blanks on their own, then having a set face and a range of expressions is clearly the way to go. If instead the character is more of a blank slate, then many different faces is the way to go.

"We're all humans." Let's try to be more!
"The chief obstacle to the progress of the human race is the human race."
"If you're falling off a cliff you may as well try to fly, you've got nothing to lose."