While Booker might be a better general election candidate, I disagree within the primaries. He is toxic to the most active and loyal segement of the Democratic coalition, and I think he's toast. Don't know much about Bullock.
Booker would probably move to the left during the campaign, but I'm not sure why you think he'd be toast in a Democratic primary? What exactly is the most active and loyal segment of the Democratic coalition and why would they have a problem with Booker? I mean, if this were the case, Sanders would have won easily in 2016, right? (And like I said, he ran against a much weaker candidate than Booker would be)
Just ftr, I don't think Booker will be the nominee anyway. Right now, I believe it will be someone like Biden.Clinton was not a particularly weak PRIMARY candidate, as she somehow amassed a huge group of people who were HUGE fans of hers; when people joked that she was "inevitable," it was kind of true. It says a lot about her (and the direction the party will move in as millenials get older, IMO) that she almost choked to Sanders and needed a lot of super delegate help, but she still had a huge group of the primary electorate that seemed to worship her. Her name recognition was through the roof, and I think the inevitability really helped her limp across the finish line. Clearly, the GE was another story. The 2016 primaries were always going to be about finding someone who could take out Clinton; 2020's will be much more crowded, and I'm inclined to believe the far left will have an easier time unifying behind someone than "the rest." Blacks have historically gotten behind one candidate (might be Booker but far from certain), but I'm not so sure about Hispanics, the union vote, women, etc.