Should I start DeShaun Foster or Fred Jackson in week 13?

Foster hasn't been very good all season. He was terrible against a great Saints run D as of late. I worry that'll cue DeAngelo Williams to take more carries. San Francisco has been pretty bad all around defensively. I don't really know much about Fred Jackson, but I do know that Buffalo runs about 20 times a game and that he is going to be the main option in place of Marshawn Lynch. Washington is middling, so any body have any suggestions?