Biden certainly has a lot of experience, but I can't get past some of his colossal judgment errors like the Iraq war. He's made more than a few blunders during his career. If anything, like Hillary, he's shown he can't look at a difficult situation and make the right call. Yes, I trust him to do the right thing a hell of a lot more than Trump, but I'm convinced Biden will depress voter turnout.
As for attacks against Sanders regarding socialism, that doesn't matter one bit. They attacked Obama constantly with labels like socialist, marxist, and communist. And Obama is practically a conservative democrat. Trump voters don't understand what any of those things are, but they'll scream them no matter who the nominee is.
The number one concern should be which nominee will most motivate turnout on the left and convince more people in the middle to vote. Sanders is easily #1 here. Hell, Yang would be better for this than either Biden, Buttigieg, and possibly even Warren. Picking another centrist/conservative dem for 2020 will just as big of a mistake as it was for 2016.