Barack is a classic BS-man with excellent public speaking skills. There's not much "there" there, if you ask me. Of course this is America, land of leased BMWs and 125% mortgages, so voters in Blue States might just "relate."
Hillary works hard behind the scenes to accomplish things that are, IMO, downright evil, and she's willing to play dirty to do it.
So, were I a leftie Democrat, I'd vote for Hillary, who is a committed authoritarian Socialist with skills.
Were I a center-left Democrat, I'd vote for Barack, because he might have a chance of winning.
As me, a South Park Republican (actually a registered and formerly active Libertarian who thinks that this isn't the time to screw around), I'm not sure.
Hillary would mobilize her opposition like no one since Abraham Lincoln, who of course saw half the country secede. 90% chance she'd lose, except against a really lousy Republican candidate like Senator RINO, AZ.
Barack, on the other hand, would be a relatively innocuous Democrat. If I'm going to have an enemy to fight, I'd rather it be a relatively weak, preppie politician with a smile and a suit than someone who actually has the wherewithal to be an enemy.
Maybe I'm underestimating Obama, but he was a Senator. It would be hard to underestimate ANY Senator's character and abilities, just like it would be hard to underestimate the postgraduate-level math skills of your average pop singer.
Now either way, Senators don't become President, generally. It could happen, if it were a Senator vs. Senator race. But otherwise, the odds are against it. Generally, Senators are bloviating BS artists, and do not show leadership qualities. That bodes poorly for an Executive candidate. Parliamentary systems work quite differently, and the PM generally doesn't have the same powers as the President of the US, either.