Alex Smith -- Jim Harbaugh is going to continue to give Alex the ability to change plays at the line, and since this is a home game against a mediocre defense I think Smith is your way to go. Big Ben might not make it through the game, he's fallen back into his slump of holding the ball too long and making bad decisions - add that up and it equals injury.
Ben is clearly the better QB, but if I were a betting man I would bet on Aelx to play the entire game and Ben to not. Alex should post *decent* numbers.
Now that said; if you are the underdog and need to do something special to win, play big ben as there is more potencial. If you are favored or about even, go with Alex as he's more of a sure thing to be "middle of the road"
Good luck this week