We'll probably never know who's actually more "electable", since only one of them (Clinton) will actually run in a GE. No doubt, plenty of people will cite "evidence" that their candidate is more electable. However, if early polls showed better numbers for Clinton, you can bet that many Clinton supporters would be pushing those numbers hard, while Sanders supporters would be dismissing them. The argument of "Sanders will drop 20 points in the polls once Republicans start attacking him" is pretty silly as well.
If Sanders had been constantly attacked for months and still managed to maintain high favorables, like Obama did in 2008 after the Potomac primary until he clinched the nomination with North Carolina and Indiana, then we should know if he is more electable or not.
