I have this exact same dilemma. I had Favre slotted in, but I think I'm going with Romo for the upside. First, the Bengals are going to run the ball into the ground. Just look at the time of possession for the last several teams playing against them. I don't think Favre will be on the field all that much. Also, look at Cinci's FP to QBs the last 6 weeks - 8, 3, 5, 11, 12, and 6.
The Chargers, on the other hand, gave up 20 FPs to Brady Quinn last week, and over 450 yards in week 10 to Philly. I think I have to go with Romo.