It's a tough choice. Biden and Bernie have very different strengths and weaknesses.
Bernie has a more adamant following and will definitely inspire more young people to vote, but he's far enough left in economics that he will turn many moderates toward Trump.
Biden has a moderate enough image that many independents will probably vote for him, but he's also pretty boring. John Kerry is a good analogy for him, and we saw how Kerry fared in 2004.
I'd go with Bernie with the hope that his economic policies would be enough to scare people towards Trump. Also, if Bernie somehow still won the presidency, his own party would probably block him from getting some of his goals achieved.