Isn't he beating every Republican candidate in the polls? I'm not so sure now but I remember reading that some time ago.
I don't know, but such comparisons are meaningless at this point. No one has been seriously attacking him on either the right or the left, whereas Clinton has been under attack since before she declared as a candidate. If the right saw him as a serious threat and started going after him over socialism, redistribution of wealth, and the tax increases needed for his proposals, you would see his negatives skyrocket - there is a lot more red meat there for conservatives than in lost emails.
The other important question is whether Sanders could mobilize Democratic minority voters, who were so crucial in the last 2 Presidential elections and who stayed home when Republicans won huge majorities in the midterms. And he is currently polling way behind Clinton with those groups.