On the Dem side, given the incredible stupidity of the GOP alienating Latinos, if Hillary is nominated she will pick Julian Castro if Trump, Cruz or Rubio is nominated. If it's an establishment GOP that's nominated I think she'll choose Evan Bayh as he would help her consolidate the Midwest and Bayhs moderate reputation would balance Clintons more liberal one. That and Bayh is well respected in Congress, is very experienced politically and has a higher trust factor that would help balance her ticket.
If Bernie wins he will either decide to go with momentum and enthusiasm as well as a partner ideologically on the same page and would take Elizabeth Warren. If he decides to balance his ticket I think he'll pick Mark Warner. Warners private business sector background and being from an important swing State would balance his ticket nicely. I think Sanders would probably balance his ticket. Two New Englanders on the same ticket is not a good idea.