Actually, I think that Biden was picked for his senate foreign relations (Iraq, Iran, Afganistan) and judicial experience (At least two Supreme court replacements) and was the harder choice. Clinton would have been the easy choice if Barack's only intention was to get elected (assuming Clinton wanted the VP slot).
Regards,
Danny
Biden was chosen to compensate for Obama's biggest weakness... lack of experience at all levels, especially on the international front. Whether Biden was on the right side of history during his career, like the Cold War, is another discussion. Either way, he definitely has a ton of experience he brings to Barack Obama's table.
McCain has a ton of 'experience' and has no need to prove anything on that front, particularly when he is competing against a freshman Senator. McCain's biggest weakness is a lack of a dynamic/spark-like element to his personality. He is steady-freddy... not exactly the best recipe for campaigning, though it does have value on the job. That's where the Palin comes in and yang's his yin, just as Biden yin's Obama's yang.