I'd prefer Bernie over Biden, but I'm not particularly confident either can beat Trump.
RCP averages:
Biden 48% Trump 45%
Sanders 48% Trump 44%
And that should be the high-water mark for Trump. What could he possibly say to get more voters on his side? He's been in our faces for five years, he's campaigning constantly, he's already shot his load.
Biden similarly is pretty much a known factor. The only fear with him is that he'll gaffe his way through the campaign, so his number legitimately could drop.
Sanders has potential to grow even more from where he's at. Like we saw with the Fox News Town Hall, moderates/conservatives like the actual Bernie more than the caricature of Bernie they get through the media. How is he going to lose support, who is a Bernie supporter now who doesn't already know exactly what he's all about? People say, "Oh, Trump will just keep repeating that Bernie's a socialist." But that's the one thing that EVERYONE knows about Bernie already. I'd love for Trump to try to lean on that message since he won't gain any ground at all with it. It's not like Trump will be the candidate to explain to everyone why his policies are superior to socialism, he can't talk policy for shyt. But he can actually grab more of the disgruntled white middle, especially in those rust belt states that Trump barely won last time, if the voters get a chance to hear his message and realize they'll actually be better off under him too.
I think winning the primary will be much more difficult for Bernie than winning the general. If he manages to overcome the cynicism about electability in the primaries then he should be the immediate favorite for November.