I've had both a Rottie and a GSD. That Rottie was an amazing, incredible dog who was badly bred and had a horrible past but was still the kindest, most intuitive boy. But I have to say I prefer German Shepherds personally... I like herding breeds! I like how biddable, eager to learn and intense they are. (ok, so I guess Rotties can herd, too)
Rotties tend to be DA, in my experience, at least to some extent. Definitely not all of them.
GSDs tend to be much more wary of strangers, and slower to warm up to people.
German Shepherds, IMO, are more energetic and physically demanding. They tend to develop neurotic behaviors if they aren't worked or exercised.
Rotties are wayyy quieter than GSDs... way, way, way.
EDIT: I didn't realize how old this thread was.