It would depend on who the nominees were. For two generic candidates, the republican odds are about 55%. In certain situations, the odds do change. Here's a few examples:
If the republicans nominate Cruz or Santorum, drop those odds to 30%. If the republicans nominate Rand Paul or Chris Christie, drop the odds to 45%. If the republicans nominate Bush, raise the odds to 60%. If the democrats nominate O'Malley, Warren, Gillibrand, or Klobuchar, raise the republican odds to 70%. If the democrats nominate Biden, drop them to 45%. If more than one of these is true, use the average of the two %'s.
(Note: Cuomo and Schweitzer would neither help nor hurt democratic odds. Walker and Kasich would neither help nor hurt republican odds.)
So your saying Joe Biden would be a stronger candidate than O'Malley, Warren, Gillibrand and Klobuchar? A guy who would be 73 on ED and makes frequent gaffes every time he speaks... Gillibrand and Klobuchar are the Dems strongest non Hillary candidates any Republican would struggle against either two.