It depends on the state of the country. If there are new economic problems, or major Democratic policies are seen as failures, the Christie/ Martinez ticket would be pretty strong.
If the economy's doing well and Obamacare is seen as a success, Clinton will probably win at slightly less than Obama 2012 levels.
Why would she ever pick McAuliffe?They're friends but she knows she can't win against a Christie/Martinez ticket with McAuliffe.
Veep candidates rarely make a difference. McAuliffe would be a swing-state Governor who clearly has a good relationship with the Clintons. It avoids stories of how a Veep candidated is untested (McCain/ Palin), doesn't get along with the top of the ticket (Kerry/ Edwards) or may be the power behind the throne (Bush/ Cheney.)