For the near term, Breaston would represent a step down from Sanders, but how big is the dropoff? If Breaston is healthy (and this is the question mark, for which I would defer to the Steelers medical staff), I don't think the dropoff is too significant. Even if there is a bit of a dropoff, it allows for improvement in other areas, which is significant.
Also, for the Steelers, the third round pick and the cap space are significant. It could be Bradshaw/Breaston/Third vs. Sanders. I don't believe that the Pats* are as tight against the cap.