Other than the Redskins game (which was also the only top10 rushing D he faced) Martin has averaged over 20 carries a game.
Yes you'd be happier if his production was better, but that's not terrible odds of him getting you ~10 points.
Expecting more than that is a pipe dream though.
Bradshaw could also get you 10 points, I don't think that choice is so clear cut really, SF D is great, but the Giants find a way to move the ball anyway, Bradshaw could get a TD.
I'd go with Martin myself, but my expectations are for ~20 touches and ~80 yards. A TD would tip it, but he's not getting goal line work.