Sanders would ensure the election against Trump would be competitive, Clinton would ensure it would not be.
Are you saying she would lose to Trump because of those ratings? Because going by that metric, Bernie should win in the biggest landslide in history rather than just competitive.
Favorability ratings really are not a good way to predict a race. There are far too many other factors at play.
I don't think that she would lose to Trump, but the question is whether she would do better than Sanders. There is no evidence that she would do better, just the mantra that she would, repeated primarily by Clinton supporters, who can hardly be objective on the matter. All I get is opinion based on gut feelings or intuition. If Clinton supporters want to cite other reasons for supporting to her, I can deal with that, but this one bothers me, because all objective evidence is to the contrary.