I would go Fred Jackson easily over Felix Jones. F Jackson is much more reliable (which is something you want in a bye week substitute) and he is more important to the BUF offense. He's also just better. I think DAL's offense will revolve around the passing game until Murray returns with Jones getting the occasional run to keep the defenses honest.
I think Jackson and Jones will get roughly the same amount of carries (Jackson shares the backfield with Spiller) but Jackson will do better because he will have more yards/carry.