Rubio.
Because youth + diversity + handsome factor > professional accomplishments + experience
Case in point: Obama vs. Hillary '08
What accomplishments did Hillary have? Though I see your point - Biden was one of the few on that stage that actually had professional accomplishments, and look how well he fared.
What were Biden's accomplishments? I guess you could argue he scuttled the Bork nomination, but that was more Kennedy's doing.
As for Rubio, there's some kind of magic line between handsome, charismatic JFK-ness and callow pretty boy. Clinton and Obama were on the right side, and Rubio is more on the Dan Quayle/John Edwards side. Not that he's dumb (Edwards wasn't either before ... you know) but he conveys zero gravitas.