My comments have to be qualified by the fact that we haven't yet been to Guy Savoy, but I have been both to L'Atelier and to the Mansion, and I have a pretty good grasp of what Guy Savoy is like based on talking to folks who have been. L'Atelier is going to be a more "casual" and quick experience, with very good food; when we ate there, we struck up a lengthy conversation with the person next to us, which was very fun (and Siegfried sat down the counter from us), but it's not terribly private or elegant. Guy Savoy is a much more traditional room and experience, and is more like the more expensive and more traditional Joel Rubochon at the Mansion. Personally, based on the menus and ambiance, I'd go with Guy Savoy, but it also depends on what else you'd like to do that evening and what your mood is.