It is really tough to sit stud players like Romo and the majority of the time it bites you in the ass. However, in your situation I would start Favre. I just dont think Romo will have a good game and obvious worst case scenario is a Pitt blitz takes him out of the game. On the other hand I think Favre will have a very nice game as they will most likely have to air it out a bit vs SF.
Ultimately its up to you and your gut feeling, good luck.