Check Out That Standard Deviation!
Many Democrats, seeing their two choices agree on most issues and are about equally popular, may vote based on which one they think has the best chance in the general election.
The way I see it, there's no way to tell. But there is a clear difference. They both have their strengths and weaknesses, but Obama is the more unknown. Hillary, love her or hate her, has been around long enough for people to have strong, even set, opinions. Who knows how the image of Obama will change once things really get going?
In other words, while they both have a decent chance of hitting 50% of the votes, Hillary's got about a 95% chance of being somewhere between 47% to 53%, while Obama's range is more 45% to 55%.