In today's NFL, you're only as good as your last game (or last season), and McFadden has the edge there. Therefore, it will be his job to lose. That doesn't mean Morris won't have a chance to take it away from him, though.
Personally, I hope DMac gets the lion's share of carries (60-70%) for the first three quarters, then Morris brings it home against worn out defense's in the fourth. Morris is better suited (physically) for that game plan to be successful. Even in going 4-12, Dallas would have won a lot more if the game was only 3 quarters long.