It's a question of judgment. One candidate selected a running mate whose experience and abilities compensated for what he lacked. The other chose a running mate without vetting, refuses to make her available for a press conference, who has been found to have violated ethics rules, who does not understand the Constitution or Bill of Rights, refuses to release medical records to prove up what should be an exceedingly simple issue, and whose selection appears to have been geared solely to wrap up the votes of those far right Republicans (the "base") who would have (probably) voted Republican regardless, as he has the proper letter next to his name.
When it comes to serving as the Chief Executive, decisionmaking is key. Senator Obama's decision in selecting Joe Biden was far and away a sounder decision, reached almost embarrassingly more pragmatically than Senator McCain's selection of Sarah Palin.
I am still undecided between Obama and Barr, as I have other concerns with Senator Obama, however, in my opinion the third option is no option at all.