Can't believe I am posting in political...but, here goes.

Back to the original question, I believe that Obama picked Biden (aside from his qualifications) for chemistry reasons. Perhaps Hillary might have brought more votes but the chemistry between Obama and Hillary (with Bill looming in the shadows) might not be good. So, Obama picked someone he is comfortable with.

McCain may face the same choice....pick a running mate he knows and is comfortable with or the most politically expedient choice without the chemistry (e.g. Romney).