"The German Shepherd is an active and large dog with a pleasant disposition, the German Rottweiler is a fierce looking and highly possessive dog that is friendly with the family members only." Seems to sum it up!
They tend to have different attitudes. I would think a German Shepherd would be more trainable ~ otherwise why would the police-force have GSD's for police dogs. You don't see many if any Rottweilers police-dogs.
Pretty good discussion on exactly this question here : ~ www.rottweiler.net
..and of course visual and physical differences :~