If it's looking like a pretty competitive race, the safer choice would be to pick Kaine in order to help lock down Virginia. If a joke Republican candidate wins (decent chance), and if it's looking like a relatively easy race for her, she should go with Castro to jumpstart the "new wave of young Democratic leaders of tomorrow."
What I don't understand is why Kaine and not Warner. Wasn't Kaine the guy in charge of the D-triple C during the 2010 rout? Why is he an attractive VP pick other than his home state? I would much rather have Warner if we were picking Senators from Virginia.